Pro-Russian separatist fighters from the so-called Battalion Vostok (East) running into position at a checkpoint in Donetsk yesterday.

Ukrainian forces regained more ground but sustained further casualties yesterday in clashes with separatists, while two Western allies urged Russia’s Vladimir Putin to exert more pressure on the rebels to find a negotiated end to the conflict.

Government forces have recently gained the upper hand in the three-month conflict against separatists in the Russian-speaking eastern regions in which more than 200 government troops have been killed as well as hundreds of civilians and rebel fighters.

The Ukrainian military says it has a plan to deliver a “nasty surprise” to the heavily-armed separatists who have dug in at Donetsk, a city of 900,000 people, after being pushed out of their bastion in Slaviansk at the weekend.

Merkel urges Putin “to exert all necessary pressure on separatists to bring them to negotiate effectively”

In a further success, military spokesman Vladyslav Seleznyov said government forces yesterday re-took the town of Siversk, east of Slaviansk, when separatists fled.

A separatist confirmed the government’s version saying it was “more or less correct”. “There was no sense in holding it and reinforcing it [Siversk] because there was a big risk of being encircled”.

But casualties mounted on the Ukrainian side with the deaths of three more Ukrainian soldiers in two attacks on Wednesday night in different parts of the east, the military said.

One was killed in an ambush of a military convoy near Luhansk, while two others died when an armoured personnel carrier was blown up by a landmine in the village of Chervona Zorya near Donetsk.

Government forces guarding Donetsk’s main international airport, scene of bitter fighting in late May, came under mortar fire yesterday but the rebel attack was repelled, Seleznyov said.

In further international diplomacy to end the worst Russia-West crisis since the Cold War, French President Francois Hollande and German Chancellor Angela Merkel urged Putin by telephone to “exert all necessary pressure on the separatists to bring them to negotiate effectively,” an Elysee Palace statement said.

They also asked him to use his influence to take concrete steps to ensure control of the border where, the Kiev government says, Russian authorities have been turning a blind eye to fighters crossing with weapons and equipment to help the rebels.

Moscow is under sanctions by the US and the EU over the Ukraine crisis but denies it is supporting the rebels in the Russian-speaking east of Ukraine.

Russia yesterday condemned an EU plan to extend the list of persons, including Russians, targeted with asset freezes and travel bans as an unfriendly move that would hinder ties with the 28-nation bloc.

The EU has agreed to add 11 new names to the list likely to take effect tomorrow, an EU diplomat said on Wednesday.

The rebellions began in April after Russia annexed Ukraine’s Crimea peninsula following the overthrow of a Moscow-backed president by mass street protests in Kiev.