(NEW YORK) -- Russian President Vladimir Putin's "special military operation" into neighboring Ukraine began on Feb. 24, with Russian forces invading from Belarus, to the north, and Russia, to the east. Ukrainian troops have offered "stiff resistance," according to U.S. officials.
The Russian military has since launched a full-scale ground offensive in eastern Ukraine's disputed Donbas region, attempting to capture the strategic port city of Mariupol to secure a coastal corridor to the Moscow-annexed Crimean Peninsula.
Here's how the news is developing. All times Eastern:
May 15, 12:35 pm
Sweden's ruling party supports a NATO bid
Sweden's ruling Social Democratic party announced Sunday that its board has decided to support the historically neutral country's bid to join NATO.
The party said it will now work to advance Sweden's application for membership to NATO.
If the application is approved by NATO, the Social Democratic Party said it will express unilateral reservations against the deployment of nuclear weapons and permanent bases on Swedish territory.
Leaders of Finland, another historically neutral Nordic country, also announced on Sunday that it will also apply for NATO membership.
-ABC News' Christine Theodorou
May 15, 12:14 pm
More people returning to Ukraine than fleeing: Ukrainian officials
Figures show the number of people returning to Ukraine in the past three days is higher than the number of people trying to flee, the State Border Guard Service of Ukraine said on Sunday.
Of the nearly 84,000 people traveling in and out of Ukraine on Saturday, more than half were Ukrainian nationals returning to the country, the Ukrainian authorities said.
More than 46,000 people returned to Ukraine on Saturday while 37,000 people left the country, the Border Guard Service said.
At least 22,000 of those who left the country traveled to Poland while the rest went to Slovakia, Hungary, Romania and Moldova, officials said.
Since the start of the Russian invasion of Ukraine, more than 6 million Ukrainians have fled the country. Since then, more than 1.6 million people have returned to Ukraine, officials said.
-ABC News' Christine Theodorou
May 15, 6:39 am
Finland confirms it will seek NATO membership
Finland's leaders on Sunday said the Nordic country would apply for NATO membership.
"It is a historic day -- of course, we have, for years, been in close partnership with NATO," Prime Minister Sanna Marin said on Sunday.
Marin and President Sauli Niinistö made the official announcement at a press conference in Helsinki, the capital. The Finnish Parliament is now expected to vote on whether to apply.
Marin said she hoped neighboring Sweden would also decide to join the military bloc in the coming days. Decisions made by both countries "will influence and affect the whole of Nordic countries," she said.
Niinistö said Finland has been discussing NATO membership internally for "at least 30 years."
"We have to keep in mind that NATO membership does not change geography, so we will always have that big border -- land and sea -- with Russians behind it," Niinistö said on Sunday.
May 15, 6:11 am
Russia 'lost momentum' in eastern offensive: UK
Russia has "lost momentum and fallen significantly behind schedule" in its offensive on eastern Ukraine's Donbas region, the U.K. Ministry of Defense said.
"Despite small-scale initial advances, Russia has failed to achieve substantial territorial gains over the past month whilst sustaining consistently high levels of attrition," the ministry said on Sunday. "Russia has now likely suffered losses of one third of the ground combat force it committed in February."
Russian forces in late March pulled out of the suburbs north of Kyiv and collected in eastern Ukraine. They began an offensive in April, but have since foundered, the U.K. said.
Low troop morale, reduced combat effectiveness and loss of equipment have all played a role in slowing the Russian advance, the ministry said.
May 13, 3:15 pm
Russian soldier accused of killing Ukrainian civilian to go on trial
Russian military commander Vadim Shishimarin, accused of killing a Ukrainian civilian in the Sumy region on Feb. 28, is set to go to trial on May 19, according to Ukraine's prosecutor general Iryna Venediktova.
The 21-year-old soldier allegedly fired his AK-47 at a car driven by a 62-year-old Ukrainian man, killing him at the scene, Venediktova said.
Shishmarin is charged with murder and two counts of robbery.
Prosecutor Andriy Syniuk said Shishmarin has cooperated throughout the investigation.
He is the first member of the Russian military to face charges, Venediktova said.
He could face a life sentence if convicted.
May 13, 1:37 pm
Biden 'underscored support for NATO's Open Door policy' on call with Swedish, Finnish leaders
On a Friday phone call with Swedish Prime Minister Magdalena Andersson and Finnish President Sauli Niinistö, President Joe Biden "underscored his support for NATO’s Open Door policy and for the right of Finland and Sweden to decide their own future, foreign policy, and security arrangements," according to the White House.
White House press secretary Jen Psaki said Thursday that the U.S. "would support" Finland and Sweden applying to join NATO "should they choose to apply."
Dmitry Polyanskiy, Russian deputy ambassador to the United Nations, told a British news podcast that Finland and Sweden officials "know the moment they become members of the NATO, it will imply certain mirror moves on the Russian side. … If there are NATO detachments in those territories, these territories would become a target -- or a possible target -- for a strike."
On Friday's call, Biden, Andersson and Niinistö "reiterated their shared commitment to continued coordination in support of Ukraine and the Ukrainian people affected by the war," according to the White House statement.
May 13, 11:30 am
Austin speaks to Russian counterpart for 1st time since invasion
Secretary of Defense Lloyd Austin spoke to his Russian counterpart, Sergei Shoigu, for the first time since the invasion of Ukraine began, according to a statement from Pentagon press secretary John Kirby.
Austin urged for an immediate ceasefire and "emphasized the importance of maintaining lines of communication," the statement said.
May 13, 3:10 am
Russian troops suffer 'significant' loss in Donbas, UK says
Russian troops suffered "significant" loss while attempting to cross a river in the disputed Donbas region of war-torn eastern Ukraine, the U.K. Ministry of Defense said Friday in an intelligence update.
"Ukrainian forces successfully prevented an attempted Russian river crossing in the Donbas," the ministry said. "Images indicate that during the crossing of the Siverskyi Donets river west of Severodonetsk, Russia lost significant armoured manoeuvre elements of at least one Battalion Tactical Group as well as the deployed pontoon bridging equipment."
"Conducting river crossings in a contested environment is a highly risky manoeuvre and speaks to the pressure the Russian commanders are under to make progress in their operations in eastern Ukraine," the ministry added. "Russian forces have failed to make any significant advances despite concentrating forces in this area after withdrawing and redeploying units from the Kyiv and Chernihiv Oblasts."
On Wednesday, the Ukrainian Ministry of Defense released images purportedly showing a wrecked pontoon crossing over the Siverskyi Donets river, with a number of damaged or destroyed armored vehicles on both banks.
"Artillerymen of the 17th tank brigade of the #UAarmy have opened the holiday season for [Russian forces]," the ministry said in a post on Twitter, alongside the photos. "Some bathed in the Siverskyi Donets River, and some were burned by the May sun."
The Ukrainian Armed Forces' Strategic Communications Directorate also tweeted images of the scene, purportedly showing the smoking wreckage after artillery units and land forces "destroyed all attempts by the Russian occupiers to cross the Seversky Donets River."
May 12, 5:11 pm
Sen. Rand Paul single-handedly delays $40B in aid, pushing vote to next week
Sen. Rand Paul, R-Ky., single-handedly sidelined the $40 billion emergency Ukraine aid bill until next week in an effort to force lawmakers to include funding for a new watchdog effort to police how the billions in taxpayer dollars are spent.
In a very rare moment, Senate Majority Leader Chuck Schumer and Senate Minority Leader Mitch McConnell joined forces.
"There is now only one thing holding us back: the junior Senator from Kentucky is preventing swift passage of Ukraine aid because he wants to add -- at the last minute -- his own changes directly into the bill. His change is strongly opposed by many members from both parties," Schumer said Thursday. "He is not even asking for an amendment … he is simply saying, 'my way or the highway.'"
But Paul stood his ground, highlighting the nearly $60 billion that the U.S. will have given to Ukraine if this package passes.
After Paul blocked an effort to expedite passage, Schumer reiterated that Paul will not get his desired changes without a vote and blamed Paul for slowing aid.
The aid bill has enormous bipartisan support and is expected to pass by sometime next week.
-ABC News' Trish Turner and Allie Pecorin
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