KIEV - Western leaders have called for a “strong reaction” from the international community to any major violation of a ceasefire in eastern Ukraine as they seek to further pressure Russia over the conflict.

The leaders of the United States, Germany, France, Britain and Italy plus EU head Donald Tusk on Tuesday also argued for strengthening a mission charged with monitoring the ceasefire and withdrawal of heavy weapons.

In video talks on the conflict, the leaders restated their support for a February 12 truce between government forces and pro-Russian separatists, the French presidency said.

“They agreed that a strong reaction from the international community would be necessary in the case of a major violation in the implementation” of the deal signed in the Belarus capital Minsk, a statement said. The leaders did not specify what a “major” break from the accord might be, nor what response it would provoke, but hinted at the possibility of further sanctions on Moscow, which the West and Kiev accuse of supporting the rebels with troops and weapons.

Germany’s Angela Merkel, France’s Francois Hollande, Britain’s David Cameron and Italy’s Matteo Renzi joined US President Barack Obama and Tusk on the video conference.

Renzi is due to meet Ukrainian President Petro Poroshenko in Kiev Wednesday to discuss the tenuous ceasefire. Both the Ukrainian army and the rebels who took up arms after the ouster of pro-Russian president Viktor Yanukovych claim to be upholding a commitment to move their artillery back from the frontline.

But the two sides also accuse each other of continuing to spring attacks in defiance of attempts to end 11 months of bloodshed that has killed more than 6,000 people according to a UN tally.

Poroshenko called Tuesday for 10 locations “where the ceasefire is constantly violated” to have permanent supervision from monitors from the Organization for Security and Co-operation in Europe (OSCE).

Ukraine said that Russian President Vladimir Putin had backed the suggestion in a four-way telephone call with Poroshenko, Merkel and Hollande on Monday. The offices of Hollande, Merkel, and Putin, in their statements on the discussion, called for intensified monitoring without listing sites.

The areas cited by Kiev as requiring close watch include the vicinity of the rebel-held airport in the separatist hub of Donetsk, as well as the village of Shyrokine, on the outskirts of the strategic port city of Mariupol.

Mariupol is the only major city in the conflict zone still in government hands.

Kiev and its allies fear the city of half a million could be the target for a new separatist offensive aimed at opening up a land bridge to the Crimea peninsula annexed by Russia a year ago.

During a visit to Tokyo Tuesday, Ukraine’s Foreign Minister Pavlo Klimkin describing the situation along the frontline as “very difficult and tense.”

The violence has nonetheless considerably abated over the past week or so, leading to cautious expressions of optimism this week from the US, Russia and NATO.

Ukraine’s defence ministry said Tuesday its biggest guns with a calibre of 100 millimetres had been “practically all” moved back from the frontline.

The separatists, by contrast, were “continuing to concentrate their forces” around areas including Mariupol, Kiev said.

Military spokesman Andriy Lysenko said the separatists had killed three soldiers in shelling since Monday, bringing to four the number of government troops reported killed this week.

“Rebels continue to increase forces in order to prepare for offensive movements” and scout territory with unmanned aerial vehicles, Lysenko added.

Top US military officer General Martin Dempsey said Tuesday he supported arming Ukraine in its battle with the rebels.

“I think we should absolutely consider lethal aid and it ought to be in the context of NATO allies because Putin’s ultimate objective is to fracture NATO,” Dempsey told senators.

President Barack Obama is weighing a possible move to provide weapons to Kiev but some NATO members - including France and Germany - are opposed to arming Ukraine over fears it could further escalate the conflict.

The OSCE meanwhile reported continuing problems gaining access to rebel-held areas, including in Shyrokine, the flashpoint village near Mariupol.

Asked about the calls for the thinly stretched mission to boost its monitoring, a spokesman for the Vienna-based rights watchdog expressed doubts.

“For more monitors and for substantial change in where they’re based (it) would require a mandate change,” OSCE spokesman Michael Bociurkiw told AFP.