The United States said Tuesday it was "deeply concerned" with a spike in violence in eastern Ukraine, where Russian-backed rebels and Ukrainian forces have clashed for three days, and called for a ceasefire.

The Ukrainian industrial hub of Avdiivka came under an unexpected assault Sunday from insurgents seeking to wrest back territory controlled by Kiev during the nearly three-year war.

"The fighting has caused dozens of Ukrainian military casualties and 10 civilian casualties. It has also left 17,000 civilians, including 2,500 children, without water, heat, or electricity," said acting State Department spokesman Mark Toner.

"To avert a larger humanitarian crisis, we call for an immediate, sustained ceasefire and full and unfettered access for OSCE monitors," he added, referring to the Organization for Security and Cooperation in Europe, which is responsible for monitoring ceasefire violations.

The State Department is still in transition, with President Donald Trump's secretary of state-designate Rex Tillerson yet to gain Senate confirmation.