(Reuters) - Fifteen people were killed in shelling in the east Ukrainian port city of Mariupol on Saturday, Ukraine's interior ministry said, an attack Kiev blamed on separatist rebels and the Russian military.

A witness described the shelling to Reuters as enough to knock the paint off his house.

The deaths follow the separatists' rejection of more peace talks and as fighting surged to its most intense in months. The United Nations said on Friday 262 had been killed in the previous nine days.

Government-held Mariupol, on the Sea of Azov, lies on a coastal route from the Russian border to Crimea, which was annexed by Russia from Ukraine last March.

The city council said rockets fired by rebels from long-range GRAD missile systems struck a multi-story building and caused fires to break out.

Oleksander Turchynov, secretary of Ukraine's national defense council, described the incident as "another bloody crime against humanity committed by the Russian military and the bands of terrorists under their complete control," in an online statement.

The attack started in the early morning, 76-year-old pensioner Leonid Vasilenko, who lives in the eastern suburbs of Mariupol, said by telephone.

"The walls were shaking, the window frames were shaking, paint started to crumble off the house. I hid in the basement. What else can you do? I took the dog and the cat. In the basement you could hear the earth tremble," he said.

The interior ministry said 15 people had been killed and 76 injured. Separatists denied responsibility for the attack, news agency Interfax reported.

Despite international calls for a ceasefire, rebel leader Alexander Zakharchenko vowed on Friday his forces would push on with a new offensive, as the UN said the conflict, which began in east Ukraine more than nine months ago, was now in its "most deadly period" since a peace deal was agreed last September.

The Kiev military reported "high-intensity" rebel attacks on government positions.

"Illegal armed groups are trying to widen the boundaries of controlled territories and correct the demarcation line to their advantage," spokesman Andriy Lysenko said in a televised briefing.

Ukrainian President Petro Poroshenko said last week Russia had 9,000 troops inside Ukraine and called on Moscow to withdraw them, blaming it for an armed aggression. Moscow denies sending forces and weapons to east Ukraine, despite what the West says is irrefutable proof.

On Friday Russian President Vladimir Putin blamed "criminal orders" by Ukrainian leaders on Friday for the surge in the conflict, which has killed over 5,000 people.

Ukraine says its troops are holding the line against the separatists after suffering a symbolic and morale-sapping setback last week when they withdrew from the main terminal at the airport in Donetsk, the biggest city in the east.

(Additional reporting by Lina Kushch; Editing by Jeremy Gaunt)

RELATED STORIES