By Lidia Kelly and Richard Balmforth
MOSCOW/KIEV, June 21 (Reuters) - Russia's Vladimir Putin on Saturday voiced limited support for Ukraine's unilateral ceasefire in its conflict with pro-Russian separatists, but told President Petro Poroshenko there had to be talks with the rebels to prevent the truce collapsing.
The Kremlin set out Putin's view after overnight fighting in Ukraine's east in which pro-Russian separatists, according to Ukrainian government forces, attacked military bases and customs posts within hours of the ceasefire coming into force on Friday night.
While welcoming Poroshenko's truce, the Kremlin said Putin believed his peace plan should not be an "ultimatum" to the rebels and warned the ceasefire would not be "viable and realistic" unless there were practical moves to start talks between the opposing sides.
Poroshenko, who has unveiled a 15-point peace plan to end the insurgency in the east, has met regional leaders and offered to respect the use of the Russian language in the east but has refused to enter into direct dialog with separatist leaders.
"The opportunity which the ceasefire opens up should be used to start meaningful negotiations and political compromise between the opposing sides in eastern Ukraine," the Kremlin quoted Putin as saying.
In an apparently conciliatory gesture, he called on "all parties to the conflict to cease fire and sit down at the negotiation table".
Poroshenko is gearing up for a diplomatic push to sell his plan but, with relations with Moscow at rock bottom and Kiev accusing Russia of fomenting the unrest, his biggest challenge will be to win over Putin.
The week-long ceasefire announced by Poroshenko came under pressure overnight when separatists attacked Ukrainian posts on the border with Russia and a military base and tried to storm an air force base, government forces said.
BORDER POSTS ATTACKED
A government forces spokesman said the separatists used mortars and sniper fire to attack Ukrainian posts at Izvareno and Uspenka on the border, wounding nine Ukrainian officers.
Rebels with large-caliber machineguns and grenade-launchers also attacked a Ukrainian position at Avdiyivka, near the main regional town of Donetsk, and a Ukrainian post at Kreminna.
Separatists controlling Slaviansk also attacked Ukrainian forces on Karachun hill overlooking the town with mortars and grenade-launchers, the spokesman, Vladyslav Seleznyov, said.
"In all these episodes, the attacks of the (rebel) fighters were deflected" without loss, Seleznyov said.
Poroshenko announced the week-long ceasefire on Friday night, urging the rebels to lay down their arms and warning that Ukrainian forces would hit back if attacked.
Ukrainian forces also repelled two attacks by around 50 heavily-armed fighters in the early morning on an air defense base at Avdiyivka, which houses surface-to-air missiles, the defense ministry said. No Ukrainian personnel were hurt.
The rebels, who have seized strategic points in major towns including Donetsk and set up "people's republics", saying they want to join Russia, insisted Ukraine had broken its own ceasefire.
Alexander Borodai, prime minister of the self-styled Donetsk People's Republic, told a news conference the ceasefire was not working and appealed to Russia to send in peacekeeping forces.
"Since last evening, combat activities are continuing. Poroshenko's artillery is bombing Slaviansk and the air force has made several raids. Words about a ceasefire as always were just that - words," Borodai said.
"The anti-terrorist operation against the people of the Donbass is in full swing," he said.
Pavel Gubarev, a prominent rebel leader, told Rossiya-24 TV channel that either Ukrainian troops were not obeying orders or Poroshenko "is lying", adding: "There is no ceasefire at all."
The insurgency in the Russian-speaking east erupted in April after street protests in the capital Kiev toppled the Moscow-backed leader Viktor Yanukovich. Russia subsequently annexed Ukraine's Crimean peninsula.
Poroshenko has offered an amnesty to separatists who disarm voluntarily as well as corridors to allow fighters from Russia or pro-Russian Ukrainian separatists to leave safely for Russia.
Ukraine, meanwhile, expressed concern on Saturday about an increase in Russian movements near the border.
"The continuing concentration of Russian armed forces and their heightened activity near the border with Ukraine causes special concern against a background of numerous facts that confirm weapons and military equipment are being supplied to the terrorists," Ukraine's Foreign Ministry said in a statement.
In Donetsk about 100 troops of the self-styled separatist Donetsk People's Republic took an oath of an allegiance, in apparent defiance of Poroshenko's peace plan.
In a ceremony on the town's Lenin Square, armed fighters, some wearing masks, pledged they would "defend the Donetsk People's Republic to the last drop of blood".
"We swear, we swear, we swear," they chanted in unison.
Alexei, a miner, said he decided to take up arms last week: "I am 43. I have children. I had a job but I dropped everything to defend the homeland."
A number of women, many in tears, rushed to hug troops and give them flowers. "A great day, a great day, we love our army. They will protect us from fascists," said Nastya, 32, holding hands with a masked soldier.
Across the square, far from the crowd, Mykola, a 23-year-old student from Donetsk, said he despised the rebels.
"They are stupid and short-sighted and brainwashed by Moscow's propaganda and common people will continue to suffer." He refused to give his full name, saying he feared reprisals for his pro-Ukrainian position. (Additional reporting by Aleksandar Vasovic and Lina Kushch in Donetsk, Pavel Polityuk in Kiev and Vladimir Soldatkin in Moscow; Writing by Richard Balmforth; Editing by Andrew Roche)