KIEV, July 10 - Ukrainian forces regained more ground but sustained further casualties on Thursday 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, Reuters reported.

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 Donetsk, a city of 900,000 people, after being pushed out of their bastion in Slovyansk last weekend.

In a further success, military spokesman Vladyslav Seleznyov said government forces on Thursday re-took the town of Siversk, east of Slovyansk, 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," he said.

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

One was killed in an ambush of an army convoy near Luhansk. Two others died when an armored personnel carrier was blown up by a landmine in the village of Chervona Zorya near Donetsk.

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

In further 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 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.

Ukrainian President Petro Poroshenko, in a phone conversation with Merkel on Thursday, said Russia was violating international norms by holding a Ukrainian woman pilot, Nadezhna Savchenko, and called for her release, his website said.

Savchenko, 33, was seized by rebels in June while she was fighting with pro-government militia near Luhansk. But she has now turned up in detention in Russia and Moscow says she has been charged with involvement in the deaths of two Russian reporters killed near Luhansk. (rt/ez)