US President Barack Obama has warned President Vladimir Putin that Russia will face greater costs if it continues its "aggressive actions" in Ukraine.

Mr Obama urged his Russian counterpart to seize the opportunity of fresh talks to find a peaceful solution to the war that has raged since April last year.

Mr Putin is due to meet with the leaders of France, Germany and Ukraine at a summit in Belarus on Wednesday.

However, there was intense fighting in eastern Ukraine ahead of the talks.

On Tuesday, four soldiers and at least eight civilians were killed in rocket attacks on a key military base and a residential area in Kramatorsk.

Government officials said the rockets were fired from a rebel-held area, but separatists denied being behind the attack.

More than 5,400 people have died since the conflict began and civilian casualties have risen in recent weeks.

Ukraine and the West accuse Russia of sending troops and arms to support the rebels, but Russia denies this.

The summit in the Belarusian capital Minsk is expected to focus on securing a ceasefire and the withdrawal of heavy weapons, as well as the creation of a demilitarised zone.

President Obama phoned Mr Putin on the eve of the talks to reiterate US support for Ukraine, just days after he refused to rule out supplying "lethal defensive weapons" to Kiev if diplomacy fails.

Russia, however, has warned the West that sending arms to Ukraine would worsen the crisis.