The U.S. said Wednesday it is sending an additional $800 million in weapons, ammunition and other security assistance to Ukraine to help it defend itself against Russia’s onslaught in the eastern region of the country.

U.S. President Joe Biden told Ukrainian President Volodymyr Zelenskyy of the new shipment in an hourlong phone conversation and later said in a statement, “The Ukrainian military has used the weapons we are providing to devastating effect.”

“As Russia prepares to intensify its attack in the Donbas region,” Biden said, “the United States will continue to provide Ukraine with the capabilities to defend itself.”

Biden’s agreement to send more weaponry to Ukraine, along with additional helicopters, came after Zelenskyy said in a video plea for more arms that “freedom must be armed better than tyranny. Without additional weapons, this will turn into an endless bloodbath that will spread misery, suffering and destruction.”

Biden said the steady Western supply of arms to Ukraine “has been critical in sustaining its fight against the Russian invasion. It has helped ensure that (Russian President Vladimir) Putin failed in his initial war aims to conquer and control Ukraine. We cannot rest now.”

Biden’s announcement came on the same day the presidents of Poland, Lithuania, Latvia and Estonia – all NATO countries bordering Russia – visited Kyiv to show support for Ukraine and one day after Putin vowed to continue Moscow’s offensive against Ukraine until its “full completion.”

The leaders of the four NATO countries, all worried that Russia could attack them if Ukraine were to fall to Moscow, traveled by train to the Ukrainian capital to meet with Zelenskyy.

While failing to capture Kyiv and much of Ukraine, Russian forces have bombarded numerous cities, killed thousands of Ukrainian civilians and destroyed housing and hospitals before Moscow pulled back its forces from western Ukraine.

A U.S. Defense Department official said Russia is regrouping its forces, including helicopters and artillery systems, in Belarus for a “renewed push” targeting eastern Ukraine.

United Nations humanitarian chief Martin Griffiths recently went to Moscow and Kyiv to seek a ceasefire. But U.N. Secretary-General Antonio Guterres told reporters Wednesday that does not look possible right now.

However, Guterres said there are “a number of proposals that were made and we are waiting for an answer from the Russian Federation in relation to those proposals –including different mechanisms for local ceasefires, for corridors, for humanitarian assistance, evacuations, and different other aspects that can minimize the dramatic impact on civilians that we are witnessing.”

Guterres said the U.N. also proposed the creation of a mechanism involving Russia, Ukraine, the U.N., and potentially other humanitarian entities, to help guarantee the evacuation of civilians from areas where the fighting is going on and to guarantee humanitarian access.

While recently acknowledging it has sustained a significant loss of troops, Moscow claimed Wednesday that more than 1,000 Ukrainian troops had surrendered in the besieged port of Mariupol. The information could not be verified.

Biden for the first time on Tuesday called Russia’s attack on Ukraine “a genocide” and contended that “Putin is just trying to wipe out the idea of even being a Ukrainian.”

Zelenskyy commended Biden’s use of the word, saying “calling things by their names is essential to stand up to evil.”

But Kremlin spokesman Dmitry Peskov rebuffed Biden’s description, saying, “We consider this kind of effort to distort the situation unacceptable. This is hardly acceptable from a president of the United States, a country that has committed well-known crimes in recent times.”

Lithuanian President Gitanas Nauseda said the European leaders visiting Ukraine planned to deliver “a strong message of political support and military assistance.”

Nauseda, Estonian President Alar Karis, Poland’s Andrzej Duda and Egils Levits of Latvia also planned to discuss ongoing investigations into alleged Russian war crimes, including the massacre of civilians.

Putin has denied Russia has committed atrocities against Ukraine and said it “had no other choice” but to invade Ukraine to protect Russian-speaking people in eastern Ukraine and to “ensure Russia’s own security.” He vowed Russia would “continue (its offensive) until its full completion and the fulfillment of the tasks that have been set,” although he did not elaborate on his end goals.

One key goal for Russia in eastern Ukraine is to take over Mariupol, a Sea of Azov port where Ukraine says thousands of residents have been killed during weeks of Russian attacks.

Russian Defense Ministry spokesman Major General Igor Konashenkov said 1,026 troops from the Ukrainian 36th Marine Brigade had surrendered at a metals factory in the city. But it was unclear when it purportedly occurred or how many forces were still defending Mariupol.

Ukraine said it is investigating a claim that a drone dropped a poisonous substance on the city, although there were no serious injuries reported. Ukrainian Deputy Defense Minister Hanna Maliar said it was possible phosphorus munitions that had been used in Mariupol.

Jeff Seldin contributed to this report. Some information also came from The Associated Press, Agence France-Presse and Reuters.