Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

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."

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

The United States "would support" Finland and Sweden applying to join NATO "should they choose to apply," White House press secretary Jen Psaki said on Thursday.

"We, of course, will respect whatever decision they make. Both Finland and Sweden are close and value defensive partners of the United States and of NATO," she said.

If the countries do apply, Psaki said, it "should be reassuring to the American people about our own security interests."

"Having a strong NATO alliance, a strong Western alliance, that's a defensive alliance, by the way, it is good for our security around the world, certainly having a strong partnership with a range of countries, including Sweden and Finland," she said.

-ABC News' Justin Ryan Gomez

Russian Deputy Ambassador to the United Nations Dmitry Polyanskiy 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," Polyanskiy warned.

He continued, "NATO is a very unfriendly bloc to us. … It means that Finland and Sweden all of a sudden, instead of neutral countries, become part of the enemy and they bear all the risks."

Despite repeated claims by Russian President Vladimir Putin that NATO’s expansion is a risk to Russian security, Polyanskiy insisted the addition of Finland and Sweden to the alliance would have little impact.

"I don’t think it will really be a blow to the security of Russia because these two states become members of NATO -- hopefully they won’t, but if they do it would be the worst solution for them, but not for Russia," he said. "Russia is ready to face NATO threats. Russia has made the necessary precautions for this."

The Russian Foreign Ministry said earlier that Russia "will be forced to take retaliatory steps both of military-technical and of other nature in order to stop the threats to its national security" should Finland become a NATO member.

While Britain has pledged military support for both Sweden and Finland if they were to come under attack, regardless of whether the countries are accepted into the alliance and under the umbrella of NATO's Article 5, the U.S. has yet to make a similar bridge security guarantee.

Testifying before the Senate Foreign Relations Committee on Thursday, Assistant Secretary of State for European and Eurasian Affairs Karen Donfried said, "We will surely find ways to assure Finland and Sweden, but the nature of that is still to be worked out."

-ABC News' Shannon Crawford