NEW YORK

UN Secretary General Ban Ki-moon said on Sunday that he is "alarmed by the severe escalation of fighting for control of Donetsk airport in eastern Ukraine."

The fighting "has already caused numerous casualties and threatened to completely unravel a hard-won September 2014 cease-fire," Ban's office said in a statement late on Sunday.

The fresh wave of violence in the pro-Russian separatist stronghold of Donetsk followed the deaths of 13 civilians who died when their bus came under rocket attack last Tuesday.

Authorities in Kiev have since launched a mass operation against the rebels, with fighting concentrated in the area in and around Donetsk airport.

"The Secretary-General stresses that further deterioration of a situation that has led to heavy loss of life and civilian suffering must be avoided at all costs," the UN statement said.

The possibility of a return to full-scale fighting in eastern Ukraine is feared amid uncertainty over how much strain a shaky September 2014 cease-fire can withstand.

More than 4,700 people have been killed since the conflict began in April 2014, according to the UN.