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

Nine humanitarian corridors are expected to open in eastern Ukraine on Monday to allow civilians escape heavy fighting, according to Ukrainian Deputy Prime Minister Iryna Vereshchuk.

She said in a statement via social media Monday that evacuation routes were agreed upon for those traveling by private cars from besieged Mariupol in the Donetsk Oblast, as well as from Berdyansk, Tokmak and Enerhodar in the Zaporizhzhia Oblast -- all of which lead to the southeastern city of Zaporizhzhia. Buses from Zaporizhzhia city were also waiting to pass a checkpoint in Vasylivka, according to Vereshchuk.

In the Luhansk Oblast, Vereshchuk said routes were established from the cities of Severodonetsk, Lysychansk, Popasna, Hirske and Rubizhne, leading to the city of Bakhmut in the Donetsk Oblast.

The United Kingdom is warning of Russia's possible use of phosphorus munitions in the besieged Ukrainian city of Mariupol.

Russian forces have already been accused of using phosphorus bombs in Ukraine since launching an invasion on Feb. 24. When deployed as a weapon, phosphorus can inflict excruciating burns and lead to infection, shock and organ failure.

After withdrawing troops from the north, the Russian military is said to be refocusing its offensive on the Donetsk and Luhansk oblasts in eastern Ukraine's disputed Donbas region, where Russia-backed separatist forces have been battling Ukrainian soldiers since 2014. Mariupol, in the Donetsk Oblast, and its residents have been under heavy Russian bombardment for over a month, but Moscow has so far failed to win full control of the strategic port.

"Russian forces prior use of phosphorous munitions in the Donetsk Oblast raises the possibility of their future employment in Mariupol as fighting for the city intensifies," the U.K. Ministry of Defense said Monday in an intelligence update.

Meanwhile, Russian shelling has persisted in the Donetsk and Luhansk oblasts, according to the ministry, "with Ukrainian forces repulsing several assaults resulting in the destruction of Russian tanks, vehicles, and artillery equipment."

"Russia's continued reliance on unguided bombs decreases their ability to discriminate when targeting and conducting strikes while greatly increasing the risk of further civilian casualties," the ministry added.

Ukrainian forces are preparing to respond to a planned Russian attack on the eastern side of the country, Ukrainian President Volodymyr Zelenskyy announced in his address on Sunday.

Russian troops are expected to move to an even larger operation in the east of Ukraine, which will enable them to carry out even more bombardments, Zelenskyy said, adding that Ukrainian forces are ready for the attack.

"We are preparing for their actions," Zelenskyy said. "We will respond. We will be even more active in providing Ukraine with weapons. We will be more active in the international arena. We will be even more active in the information field."

Zelenskyy added that he and other government officials are doing everything they can to ensure that Ukraine gets the world's attention, especially as Russia continues to attempt to influence the narrative and justify the invasion.

This coming week will be just as important as previous weeks, Zelenskyy said.

"It will be just as tense and even more responsible," he added.

-ABC News' Christine Theodorou

Nearly 23,000 Ukrainian refugees returned to Ukraine on Saturday after fleeing the country following the Russian invasion in February, according to Ukrainian and United Nations officials.

The repatriated Ukrainians are among the more than 4.5 million who left the country between Feb. 24 and April 9, according to the U.N. High Commissioner for Refugees.

More than half of the Ukrainian refugees fled to neighboring Poland, officials said.

The Polish border guard service is reporting that despite the war still raging in Ukraine, the number of refugees voluntarily returning to Ukraine reached the highest figure for a single day on Saturday since the war began, according to Ukraine's Center for Strategic Communications and Information.

The UNHCR estimated that as of April 8, more than 7.1 million people in Ukraine have been displaced due to the war.

-ABC News' Christine Theodorou