Russia has begun mobilizing troops along its border with Ukraine, presenting a fresh challenge at the Biden administration and threatening to upend a cease-fire between Ukraine and pro-Russian fighters.
The move follows escalated fighting along the demarcation line inside Ukraine, where one attack last week killed four Ukrainian soldiers and wounded two.
In response, Ukraine said Tuesday it had put its own forces on alert and was reinforcing units in eastern Ukraine and on the border with the Russian-occupied peninsula of Crimea.
While Mr. Biden has wrestled with Kremlin machinations such as a massive suspected Russian hack of U.S. government computer systems and the assessment by U.S. intelligence that Moscow attempted to interfere in the 2020 election, the situation in Ukraine is the first involving movements of Russian troops. Russia has denied meddling in the election and denied responsibility for the hack.
In Washington, a State Department spokesman said the department was discussing the situation with NATO allies.