Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty in speech denouncing West

Ukrainian President Volodymyr Zelenskyy said Tuesday he was the first foreign leader to have a conversation with new United Kingdom Prime Minister Liz Truss.

Zelenskyy said in a statement on social media that he invited her to Ukraine and the two discussed security guarantees and "coordinated further pressure" on Russia.

"The goal is to stop the aggression & bring the perpetrators to justice," he said.

A Downing Street spokeswoman said in a statement that Truss reiterated the U.K.'s "steadfast support" for Ukraine in her first call with a foreign leader since taking office.

“The leaders discussed the need to strengthen global security and the measures necessary to cut off the funds fueling Putin’s war machine," the statement said.

"She praised the Ukrainians' fight for sovereignty and self-determination and said it was essential Ukraine succeeds and Russia fails," the statement continued, adding that Truss was "delighted to accept an invitation to visit President Zelenskyy in Ukraine soon."

-ABC News' Rashid Haddou

More than 1,000 children have been confirmed killed or injured in Ukraine since Russia invaded six months ago, the humanitarian organization Save the Children said Tuesday.

At least 372 children have been killed and 635 children injured since Feb. 24 -- for an average of five children killed or injured each day, according to a Save the Children analysis of verified United Nations data.

"This grim milestone marks another dark day in this senseless war. Innocent children are being injured and killed nearly every day in Ukraine," Sonia Khush, Save the Children's country director in Ukraine, said in a statement. "Ruthless violence, including the use of explosive weapons in urban areas, has taken a big toll on children over the past six months."

"Our teams inside Ukraine continue to witness the devastating impact this war is having on children and families who have endured more than eight years of conflict. The world must act now," the statement continued.

The number of civilian casualties in the war is likely "considerably" higher, the U.N. said.

The nuclear watchdog of the United Nations is calling for the immediate establishment of a "nuclear safety and security protection zone" around the Russian-occupied Zaporizhzhia nuclear power plant in southeastern Ukraine.

The recommendation, among several others, was made in a second report released Tuesday by the International Atomic Energy Agency, which deployed an expert mission to the besieged plant last week.

"The IAEA recommends that shelling on site and in its vicinity should be stopped immediately to avoid any further damages to the plant and associated facilities, for the safety of the operating staff and to maintain the physical integrity to support safe and secure operation," the agency wrote in the report. "This requires agreement by all relevant parties to the establishment of a nuclear safety and security protection zone around the ZNPP."

The Zaporizhzhia nuclear power plant is the largest in Ukraine and all of Europe. Invading Russian forces overran the site and the surrounding town of Enerhodar in early March. The Ukrainian workers have been left in place to keep the plant operating, as it supplies electricity across the war-torn country, but the site is now on the front line between Russian-occupied and Ukrainian-controlled territory. Moscow and Kyiv have traded blame for shelling at or near the plant in recent days and weeks, fueling fears that the conflict could spark a radiation disaster.

The IAEA said it aims to maintain a "continued presence" at the plant to "help further improve and deepen the understanding of the situation."

"While the ongoing shelling has not yet triggered a nuclear emergency, it continues to represent a constant threat to nuclear safety and security with potential impact on critical safety functions that may lead to radiological consequences with great safety significance," the agency wrote in the latest report.

Crippling global sanctions are forcing Russia to turn to North Korea to secure ammunition and other vital supplies for its military, according to a newly declassified American intelligence report.

"The Russian Ministry of Defense is in the process of purchasing millions of rockets and artillery shells from North Korea for use on the battlefield in Ukraine," a U.S. official said in the report. "This purchase indicates that the Russian military continues to suffer from severe supply shortages in Ukraine, due in part to export controls and sanctions."

Given that Moscow has few viable trade partners, the official added that this could be a continuing trend.

This development, first reported by The New York Times, comes on the heels of Russia securing initial shipments of Iranian drones for use on the battlefield.

U.S. sources have released little information about the quality of the weapons from North Korea or when Russia expects to receive them, but sources within the administration said it's a sign of the Kremlin's desperation.

Sources also told ABC News that there's no indication so far that China played an indirect hand in the deal, but that they're continuing to monitor the situation closely.

-ABC News' Shannon K. Crawford