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

Israel on Monday lashed out at Russia over "unforgivable and scandalous" remarks made by its top diplomat about Nazism and antisemitism, including claims that Adolf Hitler was Jewish.

During an interview Sunday with an Italian television channel, Russian Foreign Minister Sergey Lavrov was asked about Moscow's assertion that it invaded neighboring Ukraine to "denazify" the country. Lavrov said the fact that Ukrainian President Volodymyr Zelenskyy is Jewish does not negate the Nazi elements in his country, drawing a parallel with Hitler, the chancellor of Nazi Germany.

"So when they say: 'How can Nazification exist if we're Jewish?' In my opinion, Hitler also had Jewish origins, so it doesn't mean absolutely anything. For some time we have heard from the Jewish people that the biggest antisemites were Jewish," Lavrov said, speaking to the station in Russian, dubbed over by an Italian translation.

Russia does not insist on Zelenskyy's surrender, Lavrov said, but wants the Ukrainian president to order "neo-Nazi battalions to halt resistance, lay down their arms and let civilian hostages go." Lavrov alleged that Moscow only seeks to guarantee the security of pro-Russia Ukrainians in the eastern regions.

Lavrov's comments came at a time when Israel, which was created as a refuge for Jews in the wake of the Holocaust, has sought to remain neutral amid Russia's war in Ukraine. However, Israeli Foreign Minister Yair Lapid condemned the statement made by his Russian counterpart as "unforgivable and scandalous and a horrible historical error."

"The Jews did not murder themselves in the Holocaust," Lapid, the son of a Holocaust survivor, said Monday. "The lowest level of racism against Jews is to blame Jews themselves for antisemitism."

Ukraine also denounced Lavrov's statement, with Ukrainian Foreign Minister Dmytro Kuleba saying it exposes "the deeply-rooted antisemitism of the Russian elites."

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

U.S. first lady Jill Biden will travel to Romania and Slovakia this week to meet with American soldiers, U.S. embassy staff as well as displaced Ukrainian families, the White House announced Monday.

Romania and Slovakia are hosting hundreds of thousands of refugees from Ukraine who were forced to flee their homes due to Russia's invasion.

According to a press release from the White House, Biden will depart the United States for Romania on Thursday evening. On Friday, she will visit Mihail Kogalniceau Airbase in southeastern Romania, where she will meet with U.S. military service members.

On Saturday, Biden will travel to Romania's capital, Bucharest, to meet with Romanian government officials, U.S. embassy personnel, humanitarian aid workers as well as educators who are helping teach displaced Ukrainian children. She will then travel to Slovakia's capital, Bratislava, to meet with U.S. embassy staff there, according to the White House.

On Sunday, which is celebrated as Mother's Day in the U.S., Biden will travel to the eastern Slovak city of Kosice and the small village of Vysne Nemecke, the largest of three border crossings between Slovakia and Ukraine, to meet with Ukrainian refugees, humanitarian aid workers as well as local Slovakians who are supporting the displaced families, according to the White House.

"On Mother's Day, she will meet with Ukrainian mothers and children who have been forced to flee their home country because of Putin's war," the White House said in a statement.

On Monday, Biden will meet with Slovakian government officials before heading back to the U.S.

-ABC News' Armando Garcia

At least one person -- a journalist -- was killed in a rocket attack on a residential building in Kyiv on Thursday evening, ABC News has learned.

Kyiv Mayor Vitali Kilitschko said Friday that rescuers had found the body of a victim amid the rubble.

Radio Liberty, a service of the U.S.-funded Radio Free Europe/Radio Liberty, reported that one of its employees, Vira Gyrych, was killed when a Russian missile hit her apartment in the Ukrainian capital on Thursday. Her body was found beneath the wreckage Friday morning, according to the report.

Gyrych had worked as a journalist and producer for Radio Liberty's Kyiv bureau since 2018. Prior to that, she worked for leading Ukrainian television channels, according to Radio Liberty.

"The editorial staff of Radio Liberty expresses its condolences to the family of Vira Gyrych and will remember her as a bright and kind person, a true professional," Radio Liberty said in its report.

Israeli Ambassador to Ukraine Michael Brodsky also confirmed Gyrych's death in a Twitter post, saying she was a former employee of the Israeli embassy in Kyiv.

Thursday's rocket attack came as United Nations Secretary-General Antonio Guterres visited Kyiv. Five Russian missiles flew into the city, according to Ukrainian President Volodymyr Zelenskyy. At least 10 people were injured, including four who were hospitalized, according to the Kyiv City Council.