Ukrainian authorities rushed to evacuate children from parts of the war zone Monday in the wake of a deadly rocket attack on the port city of Mariupol and as fresh clashes hit the country's east.

Another nine people were reported killed, including seven soldiers, as Kiev accused pro-Russian rebels of firing more than 100 times over the past day on both Ukrainian military positions and civilian areas.

Fighting was said to be particularly intense near the government-held city of Debaltseve, halfway between the rebel strongholds of Donetsk and Lugansk, where the military claimed separatists were attacking with tanks and rocket launchers.

International concern has mounted over the escalation in the conflict that has left a September truce in tatters, with the rebel leader in the Donetsk region last week pulling out of peace talks and vowing to capture new territory.

His comments came a day before a barrage of rocket fire on Saturday killed 30 people in a residential area of Mariupol, the last major city in the country's two separatist provinces still controlled by Kiev.

Monitors from the Organization for Security and Co-operation in Europe said the rockets were fired from the direction of separatist-held areas, though rebels distanced themselves from the attack.

The leader of the self-proclaimed Donetsk People's Republic, Alexander Zakharchenko, initially claimed Saturday to have launched an offensive aimed at taking Mariupol, but later denied ordering an actual invasion of the industrial port on the Sea of Azov. Mariupol remained calm on Monday.

The UN Security Council planned to hold a special meeting at 2000 GMT on Monday on the violence that has rocked eastern Ukraine in recent days, with Western nations calling on Moscow to rein in the insurgents the Kremlin is accused of backing.

Russia has firmly denied supplying the rebels with troops, weapons and equipment despite satellite images to the contrary and says Kiev forces have put civilians at risk by repeatedly firing on residential areas.

Monitors and aid groups say rebels have launched attacks from residential areas, prompting counter-fire from Ukrainian forces, and have warned over the risk being posed to civilians.

Pro-Kiev officials in the two separatist regions said they were planning evacuations of children from areas considered to be in immediate danger.

They included Mariinka and Krasnogorivka in the Donetsk region, with children there to be relocated to neighbouring Dnipropetrovsk province, the Donetsk police chief said.

In Lugansk, around 100 children would be moved from the village of Troitskiy, in the northern Lugansk region, where two civilians were killed in the most recent fighting, the governor said.

"We must evacuate them as quickly as possible to a secure place," governor Gennady Moskal said in a statement.

Russia is already under heavy Western sanctions over its alleged actions in Ukraine, and the recent violence has led to threats of further consequences for Moscow.

US President Barack Obama vowed to ramp up pressure on the Kremlin after Saturday's assault on Mariupol, which could form a land bridge between separatist territory near the Russian border and the Black Sea peninsula of Crimea that Moscow annexed in March.

Obama said he would look at all options -- short of military intervention -- to restrain Russian President Vladimir Putin's alleged proxy war aimed at stripping Ukraine's pro-Western leaders of their vital eastern industrial base.

In a call to Putin on Sunday, German Chancellor Angela Merkel urged the Russian president to pressure the pro-Kremlin separatists to end the upsurge in violence, her spokesman Steffen Seibert said.

Russian Foreign Minister Sergei Lavrov said Monday that Moscow would try in the coming days to facilitate contacts between Kiev and the rebels "as far as we have the powers and opportunities".

Ukrainian President Petro Poroshenko told his top generals that he had asked the European Union to tighten their sanctions on Russia at a special session of foreign ministers scheduled to be held in Brussels on Thursday.