Russia-Ukraine updates: US sanctions Russian military shipbuilder, diamond miner

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

There is an unexploded missile in the immediate vicinity of the nuclear research facility in Kharkiv, Ukraine's regulatory authority told the International Atomic Energy Agency.

The IAEA said it is continuing to monitor the situation.

"The regulator confirmed reports of an unexploded rocket of the multiple launch rocket system 9K58 Smerch 'detected in the immediate vicinity' of a nuclear research facility that has previously been damaged by shelling," the IAEA said in a statement.

It added: "the regulator said 'constant shelling' of the area meant that no measures had yet been taken to dispose of the rocket."

The facility is used for research and development and radioisotope production for medical and industrial applications. The site's nuclear material cannot sustain a fission chain reaction and the radioactive inventory is low, according to the IAEA.

Eight of Ukraine's 15 operational reactors, at four sites, are continuing to operate, including two at the Zaporizhzhya power plant. The other reactors are shut down for regular maintenance, the IAEA said.

-ABC News' Guy Davies

Ukraine's prosecutor's office said that 136 children have been killed in the war and 199 have been injured, as of Saturday morning local time.

The grim update comes on Day 31 of Russia's invasion of Ukraine.

-ABC News' Guy Davies

The Russian army is reluctant to engage in large-scale infantry operations, the British Ministry of Defense said Saturday.

"Russia continues to besiege a number of major Ukrainian cities including Kharkiv, Chernihiv and Mariupol. Russian forces are proving reluctant to engage in large scale urban infantry operations, rather preferring to rely on the indiscriminate use of air and artillery bombardments in an attempt to demoralize defending forces," the Ministry of Defense said in a statement posted online.

It added: "It is likely Russia will continue to use its heavy firepower on urban areas as it looks to limit its own already considerable losses, at the cost of further civilian casualties."

-ABC News' Guy Davies