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

About 12 Russian missiles struck in and around the Kremenchuk oil refinery in central Ukraine, according to Dmytro Lunin, head of the Poltava Regional Military Administration.

No one was injured and crews have extinguished the blaze, Lunin said.

Most missiles hit the infrastructure of the refinery, which is not operating, Lunin said.

-ABC News' Irene Hnatiuk and Yuriy Zaliznyak

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan