Six soldiers have been killed in fighting and a landmine explosion has injured a Russian journalist in rebel-held eastern Ukraine as a flare-up of violence undermined a shaky two-month ceasefire.

Russian TV network Zvezda, which is controlled by the country's defence ministry, said one of its journalists was badly injured in the village of Shyrokyne on the outskirts of the strategic port city of Mariupol on Tuesday after stepping on a landmine. An AFP journalist at the scene said the reporter was with the rebels when the incident occurred.

Meanwhile, Andriy Lysenko, the Ukrainian military spokesman, said six soldiers had been killed and 12 injured in a surge of fighting in the previous 24 hours.

The heaviest fighting took place around the airport of the rebel bastion of Donetsk, a city once home to a million people in the industrial east.

He said the situation in the conflict zone "remains unstable", with "armed provocations by the enemy continuing on almost all sides".

The deaths, the highest toll in the last 10 days, come as the foreign ministers of Ukraine, Russia, Germany and France expressed "grave concern" over surging violations of the February ceasefire.

The top diplomats from the four countries held talks in Berlin overnight to evaluate the status of the truce deal aimed at ending a year of fighting that has killed more than 6,000 people.

Talks on the situation in Ukraine are expected to continue at a meeting of G7 foreign ministers in the northern German city of Luebeck on Tuesday and Wednesday, attended by US Secretary of State John Kerry.

Pentagon concerned

Russian forces are still operating inside Ukraine and have set up training camps for pro-Moscow separatists,
despite a ceasefire agreement backed by the Kremlin, the Pentagon said on Tuesday.

"There is no question that the Russians continue to operate inside of Ukrainian territory," Colonel Steven Warren, the Pentagon spokesperson, said.

The United States believes the Russians "have established training centres in Ukraine to train separatist fighters in tasks such as gunnery and artillery firing drills," Warren said.

"We also know that they continue to maintain advanced surface-to-air missile systems near the front line" in the east of the country, which violated a ceasefire accord, he added.

There are also Russian "command and control elements" helping to coordinate military operations for the separatists, according to Warren.

Russia has denied widespread reports and allegations of a military role in eastern Ukraine.

Source: AFP