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

White House Press Secretary Jen Psaki told reporters Tuesday the Biden administration is still weighing additional sanctions against Russia that could be announced by the end of this week.

"I think that we're still ... doing considerations and running a process for additional sanctions, but there are some under consideration. Yes," Psaki told reporters aboard Air Force One.

Psaki also provided a readout of President Joe Biden's call with U.S. allies Tuesday morning, saying the video conference was an opportunity for the world leaders to continue discussing their ongoing efforts to support Ukraine as Russia continues to carry out its assault against the country.

"The leaders reiterated their commitment to continue providing security, economic and humanitarian assistance to Ukraine in its time of need," Psaki said.

"The leaders affirmed their solidarity with the Ukrainian people and condemned the humanitarian suffering caused by Russia's unprovoked and unjustified invasion," Psaki added. "They also discussed their respective diplomatic engagements and their coordinated efforts to continue to impose severe economic costs to hold Russia accountable."

Psaki also said that the United States will continue to provide Ukraine with more ammunition, military assistance and money.

-ABC News' Mariam Khan

U.N. Secretary-General Antonio Guterres is calling for a four-day halt in fighting in Ukraine to coincide with Orthodox Easter.

Guterres said Tuesday that he is asking that a cease-fire begin on Thursday as religious observances get underway.

"I urge all parties -- and all champions of peace around the world -- to join my Easter appeal. Save lives. Stop the bloodshed and destruction. Open a window for dialogue and peace," Guterres said in a video he posted on Twitter.

The U.N. chief said Orthodox Easter is coming amid an intensifying Russian offensive in eastern Ukraine, making a “humanitarian pause” in the fighting all the more urgent.

Guterres said the goal is to allow for the evacuation of civilians from “current or expected areas of confrontation” and to get more humanitarian aid into places where it is desperately needed such as Mariupol, Donetsk, Luhansk and Kherson. More than 4 million people in those areas need assistance, Guterres said.

There was no immediate response from Russian and Ukrainian leaders.

-ABC News' Alexandra Faul

The United States has seen "limited" Russian offensive operations southwest of Donetsk and south of Izium in eastern Ukraine, a senior U.S. defense official said Tuesday, describing the military activity as "preludes to larger offensive operations that the Russians plan to conduct."

"These are actual ground offensives, and they are being supported, of course, by some long-range fires, mostly artillery, which is right out of the Russian doctrine," the official said.

But while there is ongoing fighting in the region, a more devastating offensive strategy appears to be still in the works, according to the official.

Both Ukrainian President Volodymyr Zelenskyy and Russian Foreign Minister Sergei Lavrov have said Russia's new offensive has started in the Donbas region. But the U.S. defense official said it does not appear the new offensive has begun in earnest.

"We think that these offenses are preludes to larger offensive operations that the Russians plan to conduct," the official said. "So we're not pushing back on the notion that offensive operations have begun, but again, we think that this is a prelude of larger offensive operations that are potentially still in the offing here."

The Pentagon believes Russia's military is assessing the mistakes it made in fighting in the north, where it was plagued with logistical and supply problems. U.S. defense officials suspect Russia is conducting what they describe as "shaping operations" to set favorable conditions on the battlefield before beginning its new offensive in earnest.

"In other words, continue to reinforce, continue to make sure they have logistics and sustainment in place, continue to make sure that they have proper aviation and other enabling capability," the official said.

Over the last 24 hours, two Russian battalion tactical groups comprised of about 2,000 combat troops have been sent into Ukraine, according to the official. It's now estimated that 78 Russian BTGs are inside the country, all in the south and east.

-ABC News' Matt Seyler

Polish Prime Minister Mateusz Morawiecki visited western Ukraine Tuesday to mark the opening of a temporary housing community his country donated for Ukrainians displaced by the war.

“This is short term container housing. Conditions here are typically temporary, just for those people who just have nowhere to go,” Morawiecki said during his visit to Lviv.

Morawiecki said Polish teams would also be going to Bucha and Kyiv Tuesday to build more temporary houses.

“Russia is not only ruining all foundations of people’s life in Ukraine, it burns everything to plain concrete and only smoke and ash remains," Morawiecki said.

The prime minister said nearly 10 million people have been forced to leave their homes in Ukraine since the start of the Russian invasion on Feb. 24 and 2.5 million of the displaced residents have sought refuge in Poland.

"But we have to remember that approximately 6 million are still staying somewhere in Ukraine," Morawiecki said.

-ABC News' Yuri Zalizniak