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

U.S. Embassy staff returned to Ukraine for the day on Monday, marking the first trip back in the country since February.

"We expect to continue to do day trips for the next week or two and we very much hope that the conditions will permit us to go back to Kyiv by the end of the month," Kristina Kvien, the U.S. chargé d'affaires to Ukraine, said in a statement.

Kvien said, "The message to Russia is: you failed -- Ukraine is still standing, the government is still functioning and we are going back to Lviv first and then Kyiv to help the government."

Kvien continued, "We are listening to the security professionals and when they tell us we can go back we go back. And while we are eager to do so we also want to make sure we are listening to the experts. So, the fact that we are here in Ukraine means that the security officials just said that it is ok and safe to meet here in Lviv and hopefully we will get the clearance to go back to Kyiv."

-ABC News' Clark Bentson

Dozens of civilians trapped for weeks inside a steel plant in the devastated Ukrainian city of Mariupol were expected to reach Zaporizhzhia on Monday, according to Ukrainian President Volodymyr Zelenskyy.

In a statement posted to Twitter on Sunday, Zelenskyy said a first group of about 100 people were already en route to the Ukrainian government-controlled city, about 140 miles northwest of Mariupol.

"Tomorrow we'll meet them in Zaporizhzhia," Zelenskyy tweeted. "Grateful to our team!"

Many more civilians remain trapped at the sprawling Azovstal Iron and Steel Works plant in Mariupol -- the last holdout of Ukrainian resistance to Russia's bombardment of the strategic southeastern port city -- which Russian forces resumed shelling overnight.

"Today, for the first time in all the days of the war, this vitally needed green corridor has started working," Zelenskyy said Sunday in his nightly address.

A pair of "powerful explosions" were heard early Monday in the western Russian city of Belgorod, about 15 miles from the border with Ukraine, according to the regional governor.

"I woke up to the sound of two powerful explosions half an hour ago. According to the anti-crisis center, there were no reports of casualties or damage. Footage showing flashes in the sky has emerged on social media," Belgorod Oblast Gob. Vyacheslav Gladkov said in a statement posted on Telegram.

The blasts followed a series of other explosions and fires at industrial and military facilities across Russia in recent weeks. On Sunday, the governor of Russia's western Kursk Oblast, which also shares a border with Ukraine, said a railway bridge used to transfer Russian troops to Ukraine had partially collapsed. In a video posted on Telegram, Kurk Oblast Gov. Roman Starovoit blamed the incident on sabotage.

-ABC News' Edward Szekeres, Max Uzol, Irene Hnatiuk and Fidel Pavlenko

Over a quarter of Russian military units committed to fight in Ukraine have been likely rendered "combat ineffective," the U.K. Ministry of Defense said Monday in an intelligence update.

"At the start of the conflict, Russia committed over 120 battalion tactical groups, approximately 65% of its entire ground combat strength," the ministry said. "It is likely that more than a quarter of these units have now been rendered combat ineffective."

Meanwhile, some of Russia's most elite units, including the Russian Airborne Forces or VDV, "have suffered the highest levels of attrition," according to the ministry.

"It will probably take years for Russia to reconstitute these forces," the ministry added.

On Sunday, Ukrainian presidential adviser Oleksiy Arestovych said at least 30 senior Russian military officers have been eliminated in the previous five days.

-ABC News' Edward Szekeres, Max Uzol, Irene Hnatiuk and Fidel Pavlenko