Ukrainian President Volodymyr Zelenskyy made a surprise video appearance at the Doha Forum in Qatar, addressing the energy-rich nation directly, calling on it and other energy-producing countries to increase energy production.

"The responsible states, in particular the State of Qatar, are reliable and reputable suppliers of energy resources. And they can contribute to stabilizing the situation in Europe. They can do much to restore justice," Zelenskyy said.

Zelenskyy added, "The future of Europe depends on your efforts! I urge you to increase energy production! So that Russia understands that no state should use energy as a weapon to blackmail the world."

Zelenskyy also warned that a food crisis will come after the migration crisis.

"Wheat, oil, corn and other agricultural products from our country are the basis of stability and internal security of many countries in different parts of the world… Russian troops mine fields in Ukraine, blow up agricultural machinery, destroy fuel reserves needed for sowing. They blocked our seaports," Zelenskyy said.

Zelenskyy added: "Our state will have enough food. But the lack of exports from Ukraine will hit many nations in the Islamic world, Latin America and other parts of the world. Where some invaders still dream of going to strengthen their old privileges."

Zelenskyy also drew direct comparisons between the destruction of Mariupol and the Russian bombing of Aleppo.

He criticized Russia for threatening the world with nuclear weapons, and called on countries to boost their production to counteract the global dependence on Russian oil.

-ABC News' Guy Davies

President Joe Biden and Polish President Andrjez Duda delivered brief remarks ahead of their expanded bilateral meeting, with both sides stressing their strong relationship, and the importance of unity in the midst of Russian aggression against Ukraine. Biden is set to give a speech in Warsaw later Saturday.

Biden recalled a previous trip he made to Warsaw 25 years ago, after advocating for Poland to Join Nato, and said his message then is the same as today: "For your freedom and for ours."

Biden also reiterated the U.S.'s commitment to NATO's Article 5.

"The single most important criterion in this time-changing world -- so much has changed, not just here, but in other parts of the world -- is that NATO's stay absolutely, completely, thoroughly united. [There needs] to be no separation, and our points of view, and whatever we do, we do in unison, and everyone, everyone comes along," Biden said.

Biden added: "I'm confident that Vladimir Putin was counting on being able to divide NATO, and being able to separate the Eastern flank from the West. To be able to separate nations based on past histories. But he hasn't been able to do it. We've all stayed together. And so I just think it's so important that we, Poland and the United States, keep in lockstep the power pursuit."

Biden also spoke about Poland's work on the humanitarian side of the crisis, praising the country for taking in so many refugees, but acknowledging that it should not be just on Poland to handle the brunt of the load.

"We do acknowledge that Poland is taking on a significant responsibility that I don't think should just be Poland, It should be the whole world -- all of NATO's responsibility. The fact that you have so many, so many Ukrainians seeking refuge and this country of Poland," Biden said.

He added: "We understand that because we have on our southern border thousands of people a day --literally, not figuratively --trying to get into the United States. But we believe that we the United States, should do our part relative to Ukraine as well by opening our borders to another 100,000 people. And with that, and in addition to that, I think it's important that we are in constant contact, about how we each wish to proceed, relative to what Russia is doing, and how to proceed."

-ABC News' Molly Nagle

President Joe Biden arrived at the Polish presidential palace Saturday for a bilateral meeting with Polish President Andrjez Duda, as the Russian invasion of Ukraine continues.

Biden and Duda shook hands and stood shoulder to shoulder as both countries' national anthems played.

Biden was met by three branches of the Polish military, as the two leaders reviewed the troops. They then headed inside for the meeting.

Biden was joined by a delegation of seven U.S. officials, including Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin, who participated in the meeting, according to the White House.

The Polish delegation consisted of six officials, including the ministers of foreign affairs and national defense, according to the White House.

-ABC News' Molly Nagle

President Joe Biden dropped by a meeting between U.S. Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin and their Ukrainian counterparts Saturday morning.

The meeting between the secretaries lasted for an hour and 46 minutes, according to the State Department.

The two secretaries pledged continued U.S. economic, military and humanitarian aid to Ukraine and provided the Ukrainians updates after Thursday's NATO, EU and G-7 summits.

"The Secretaries and Ministers discussed outcomes of the March 24 extraordinary NATO Summit in Brussels, and the United States’ unwavering commitment to Ukraine’s sovereignty and territorial integrity in the face of the Russian Federation’s increasingly brutal assault on Ukrainian cities and civilian population," said Ned Price, a spokesperson for the Department of State, in a statement.

In a statement from the White House, Biden reaffirmed the U.S.'s commitment to Ukraine, saying the officials discussed further efforts to help Ukraine defend its territory.

"The President updated the Ministers on U.S. efforts to rally the world in support of Ukraine’s fight against Russian aggression, including through the President’s meetings this week in Belgium, and the significant military and humanitarian assistance the United States is providing to Ukraine," the White House said in a statement.

The White House added: "They also discussed the United States’ ongoing actions to hold President Putin accountable for Russia’s brutal aggression, in coordination with our allies and partners, including through the new sanctions actions announced by the President in Brussels on March 24.

-ABC News' Conor Finnegan and Sarah Kolinovsky

All Russian troops have left the Ukrainian cities of Kyiv and Chernihiv, withdrawing north toward the borders of Belarus and Russia to consolidate before likely redeploying to the Donbas region in eastern Ukraine, a senior U.S. defense official told reporters Wednesday.

But even with the Russians gone, the territory remains treacherous.

"There are some indications that they left behind mines and things like that, so the Ukrainians are being somewhat careful in some areas north of Kyiv as they begin to clear the ground and clear the territory and re-occupy it," the official said.

While the U.S. hasn't yet seen these troops redeploy elsewhere in Ukraine, it'll likely happen soon, according to the official. Ukrainian forces are preparing for a major fight in Donbas, the official said.

The official also said the Pentagon is "monitoring" an apparent nitric acid explosion in Ukraine's Luhansk region, which Russia blamed on Ukraine.

"We've seen the Russians claim that this was a Ukrainian attack on this. We do not believe that is true," the official said. "We do believe that the Russians are responsible, but exactly what they used when they did it, why they did it, what the damage is, we just don't have that level of detail," the official said.

The official also noted that a small number of Ukrainians currently in the U.S. for "professional military education" were pulled aside for a couple days of training on Switchblade drones, which the U.S. is sending overseas as part of its military aid, according to the official.

"Although it's not a very difficult system to operate, we took advantage of having them in the country to give them some rudimentary training on that," the official said.

-ABC News' Matt Seyler