Kerry slams rebel attack on Ukraine port city

US Secretary of State John Kerry has condemned the rebel attack on a civilian neighbourhood in southeastern Ukraine that killed at least 30 people.

"I join my European counterparts in condemning in the strongest terms today's horrific assault by Russia-backed separatists on civilian neighbourhoods in Mariupol," Kerry said in a statement Saturday, according to a Xinhua report.

He accused Russia of an "irresponsible and dangerous decision" of starting the supply of advanced weaponry to the rebels again, and urged the Vladimir Putin government in Russia to close the country's international border with Ukraine and withdraw all weapons, fighters and financial backing.

"Otherwise, the US and international pressure on Russia and its proxies will only increase," Kerry's statement said.

On Saturday, the government-controlled Ukrainian port city of Mariupol came under rocket fire, which hit an open-air market and nearby homes and shops and caused heavy casualties.

The toll in the attack has risen to 30, while almost a hundred people have been wounded, according to local authorities.