U.S. and European allies “will not tolerate any aggression against Finland or Sweden” as their applications to join NATO are being considered, White House national security adviser Jake Sullivan warned Wednesday.

President Joe Biden said the U.S. would “remain vigilant against any threats to our shared security, and to deter and confront aggression or the threat of aggression.”

Sullivan was asked to clarify if that meant the U.S. was extending NATO security protections to Finland and Sweden during this time, and he said Article 5 only kicks in when all 30 allies ratify the accession.

“But the United States, is prepared to send a very clear message, as are all of our European allies, that we will not tolerate any aggression against Finland or Sweden during this process, and there are practical measures that we can take along those lines that Secretary [of Defense Lloyd] Austin will coordinate with his counterparts about Finland and Sweden," Sullivan told reporters.

With Turkey opposed to this move, Sullivan told ABC News' MaryAlice Parks that the White House is “confident at the end of the day” that Finland and Sweden “will have an effective and efficient accession process” and that “Turkey's concerns can be addressed.”

Biden will host the leaders of Sweden and Finland at the White House Thursday.

“Two nations with a long tradition of neutrality will be joining the world's most powerful defensive alliance, and they will bring with them strong capabilities and a proven track record as security partners and President Biden will have the opportunity to mark just what a historic and watershed moment this is when he meets with them tomorrow," Sullivan said.

-ABC News' Justin Gomez and MaryAlice Parks

Ahead of a meeting at the United Nations, Secretary of State Antony Blinken met with Turkish Foreign Minister Mevlut Cavusoglu on Wednesday.

Blinken told reporters he was grateful for the solidarity Turkey has shown against Russian aggression.

While Cavusoglu said he would work with Blinken to "overcome the differences through dialogue and diplomacy," he signaled that Turkey still had significant reservations about Sweden and Finland joining NATO, complicating their path to membership.

"Turkey has been supporting the open door policy of NATO even before this war, but with regards to these possible candidates—already candidate countries—we have also legitimate security concerns that they have been supporting terrorist organizations, and there are also export restrictions on defense products," Cavusoglu said.

Then adding, "We understand their security concerns, but Turkey’s security concerns should be also met."

Turkey has expressed concerns about Finland and Sweden joining NATO over the countries' support of the Kurdistan Workers' Party, or PKK, which the Turkish government considers a terrorist organization.

-ABC News' Shannon Crawford

Russian offensive operations in Donbas are becoming more modest, shrinking both in size and scale, according to a senior U.S. defense official.

The Russians are making little progress so far in Donbas, with lots of back-and-fourth between both sides, according to the official.

"We see them hew very closely to their doctrine of artillery fire then a font of frontal attack by formations that are small, and in some cases, not fully resourced, fully manned, fully strong. And they get rebuffed by the Ukrainians," the official said.

Russian forces are also still suffering from poor communication between commanders and are having other coordination issues, according to the official.

To the northeast of Kharkiv, Ukrainian forces continue to push Russian troops back toward their border, according to the official.

Russian forces are making some progress pushing closer toward the Black Sea from between Kherson and Mykolayiv, according to the official. The official said it is not clear what the intent is for this line of advance, but the U.S. sees no signs of an imminent naval assault at this time.

The U.S. believes Russia is "certainly trying" to disrupt to flow of military aid moving through Ukraine, but there have been no indications that it has had any success, according to the official.

Three of the eleven Mi-17 helicopters, more than 200 of the 300 Switchblade drones and nearly 10 Phoenix Ghost drones that the U.S. has promised Ukraine have been delivered, according to the official. The Ukrainians have told the Pentagon that 79 of the 90 U.S. howitzers that were delivered are now being used in combat.

-ABC News' Matt Seyler

U.S. Secretary of State Antony Blinken announced Wednesday that the U.S. is officially resuming operations at its embassy in Kyiv.

"The Ukrainian people, with our security assistance, have defended their homeland in the face of Russia’s unconscionable invasion, and, as a result, the Stars and Stripes are flying over the Embassy once again," Blinken said in a statement.

The U.S. has put in place additional measures to increase the safety for its staff and enhanced its security measures and protocols.

"We are committed to confronting the challenges ahead," Blinken said. "We reaffirm our commitment to the people and government of Ukraine, and we look forward to carrying out our mission from the U.S. Embassy in Kyiv."

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