Ukraine and Russia are building up their military forces in the eastern Ukrainian region of Donbas as Moscow switches the focus of its offensive.
The coming period is "crucial" for Ukraine, Western officials say. President Zelensky says Russia is concentrating tens of thousands of soldiers as it prepares for its new assault.
In a rare public appearance, President Vladimir Putin has described the Russian military's goals in Ukraine as "noble."
Russian officials have said that the focus of their forces is now the "complete liberation" of the Donbas, which broadly refers to Ukraine's eastern regions of Donetsk and Luhansk, where Russian-backed separatists held significant territory before the invasion.
Western officials say they expect Russia to double or even possibly triple its troops numbers in the Donbas. Reports and satellite images suggest a build-up of Russian forces is under way on Ukraine's border, in Russia's Belgorod, Voronezh and Rostov regions.
Reinforcements are also building up in the Kharkiv region to support the Russian push south. They are hoping to advance down from Izyum towards Slovyansk and encircle Ukrainian forces, according to the Institute for the Study of War.
Ukrainian commanders and Western officials say Russia has continued to shift its forces from areas north of Kyiv, and from Belarus, toward Kharkiv and the Donetsk and Luhansk regions, where fighting has been ongoing since 2014.
Officials expect Russia to mount a new offensive possibly with the goal of taking the remaining territories of the Donetsk and Luhansk regions that are still controlled by Ukrainian government forces.
The death toll from the conflict, which erupted when Russia invaded Ukraine on February 24, remains unclear.
Ukrainian commanders said on April 12 that 19,500 Russian troops had been killed to date. The most recent official Russian Defense Ministry tally, in late March, was about 1,370.
U.S. officials say publicly that Russia has lost at least 10,000 troops, and privately, U.S. and Western officials put the figure closer to the Ukrainian estimate -- which would mean Russia has lost more troops in Ukraine than it did in the 10 years of the Soviet war in Afghanistan.
Ukraine has not released its own casualty figures, saying it is a state secret, but President Volodymyr Zelenskiy said publicly in mid-March that about 1,300 Ukrainian soldiers had been killed.
A U.S. Defense Department official said on April 11 that Russian forces were reinforcing around the Donbas region, including near the town of Izyum, which reportedly fell to Russian troops last week.
A Russian convoy including command and control vehicles, and infantry and helicopter support units, was seen heading for Izyum, the official said.
The official also said more than 60 Russian battalion tactical groups – each numbering between 800 and 1,000 troops – were believed to have deployed from Kherson in south central Ukraine, to the northeast through the Donbas, up to the Russian border.
Russia has also appointed a new general to take unified command of the Donbas offensive, a move Western officials said appeared to be aimed at resolving the confusion and missteps that plagued Russian forces in the first phase of the war.
