By Richard Balmforth and Maria Tsvetkova
KIEV/DONETSK Ukraine (Reuters) - 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.
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 in Donetsk, a city of 900,000 people, after being pushed out of their bastion in Slaviansk last weekend.
In a further success, military spokesman Vladyslav Seleznyov said government forces on Thursday 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," 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 armoured 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.
Moscow protested to Kiev on Thursday after it said Ukrainian military fired on a Russian border checkpoint. The Foreign Ministry said it was not the first time the border post at Gukovo had come under fire.
Moscow is under sanctions by the United States and the European Union over the Ukraine crisis but denies it is supporting the rebels in the Russian-speaking east of Ukraine.
Russia condemned an EU plan to extend a list of people, including Russians, targeted with asset freezes and travel bans as an unfriendly move that would hamper ties with the 28-nation bloc. The EU has decided to add 11 new names to the list, likely to take effect on Saturday, an EU diplomat said.
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.
The rebellions began in April after Russia annexed Ukraine's Crimea peninsula following the overthrow of a Moscow-backed president amid mass street protests in Kiev.
Ukraine's border guard service reported that six Russian helicopters had flown some distance over the border into Ukraine on Wednesday and then returned home in what it described as "direct provocations in support of the terrorists".
In its version of the call between Putin, Hollande and Merkel, the Kremlin said they had agreed on the need for a "swift renewal" of the ceasefire and another round of peace talks involving the "contact group" and separatist leaders.
The group includes a former president of Ukraine, informally representing the Ukrainian government, Moscow's ambassador to Kiev and a high-ranking member of the security watchdog, the Organisation for Security and Cooperation in Europe.
Poroshenko refused to prolong a 10-day unilateral ceasefire when it expired on June 30 and ordered a resumption of the military offensive against the rebels resulting in Slaviansk being re-taken at the weekend.
His security chiefs say the rebels repeatedly breached the government's unilateral ceasefire, resulting in many Ukrainian military deaths, and have ruled out any more such truces until the rebels lay down their arms.
Rebels in Donetsk, commanded by a Russian military adventurer called Igor Girkin or Strelkov, say they are recruiting new fighters to resist government forces despite being now without help from Russia.
Buoyed by the success in Slaviansk, Ukrainian security officials say they have a plan ready to crush the rebels in Donetsk, though Poroshenko has ruled out air strikes and artillery bombardment because of the large civilian population.
The rebels are occupying administrative buildings in both Donetsk and Luhansk and are dug in on the outskirts of Donetsk.
Thousands of people have fled Donetsk, a major industrial hub and Euro-2012 soccer venue, since the onset of the conflict and many businesses are closing down.
With armed men out on the streets, tension is growing. Several social groups came together on Thursday to urge citizens to join a daily prayer session at noon everyday from next Monday.
(Additional reporting by Natalia Zinets and Thomas grove in Kiev and Alissa de Carbonnel in Moscow; Editing by Robin Pomeroy)