Ukrainian Interior Minister Arsen Avakov is urging people in the Donetsk region of eastern Ukraine to stay indoors because of fighting between government forces and pro-Russian separatists.
Avakov gave the warning Tuesday to those in Slovyansk and the nearby cities of Kramatorsk and Krasny Liman.
The violence Tuesday comes a day after hundreds of heavily armed separatists attacked a government facility near the Russian border, triggering a daylong battle that left as many as seven people dead.
The fighting near Luhansk, one of the largest battles of the two-month separatist insurgency, sparked renewed U.S. criticism of Russia's role in increasingly violent eastern Ukraine.
The U.S. State Department accused Russia of supporting the insurgency with Russian fighters and weaponry, and said Moscow has done "nothing to stop" such cross-border transfers.
Spokeswoman Jen Psaki spoke Monday just hours after Russian Foreign Minister Sergei Lavrov announced a United Nations draft resolution seeking the establishment of "humanitarian corridors" that would guarantee unhindered access to the region for humanitarian aid.
In other developments, the White House said Vice President Joe Biden will lead a delegation to Kyiv later this week for the inauguration of President-elect Petro Poroshenko.
A statement said Biden also will confer Saturday with Ukrainian officials on how Washington can help Kyiv stem government corruption, strengthen democratic institutions, and foster sustainable economic growth.