* Ceasefire to run for a week until June 27
* But President says Ukraine forces will fire back if attacked
* Peace plan offers "corridors" to Russia for rebel fighters (Adds rebel calling for Russian peacekeepers paras 10-12)
By Richard Balmforth and Aleksandar Vasovic
KIEV/DONETSK, Ukraine, June 21 (Reuters) - Pro-Russian separatists attacked Ukrainian posts on the border with Russia and a military base and tried to storm an airforce base overnight into Saturday, government forces said, putting a Ukrainian unilateral ceasefire under pressure.
The action came just hours after the start of a ceasefire at 10 p.m. on Friday by Ukrainian forces, ordered by President Petro Poroshenko as part of his plan to end the rebel insurgency in the east of the country.
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 big calibre machine guns 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 defence base at Avdiyivka, which houses surface-to-air missiles, the defence 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, said Ukraine has 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,"
Poroshenko, installed on June 7 as president after seven months of turmoil in the ex-Soviet republic, ordered government forces to cease firing to allow his 15-point peace plan to take root. The ceasefire ends at 10 p.m. on June 27.
The insurgency in the Russian-speaking east erupted in April after street protests in the capital Kiev toppled the Moscow-backed Viktor Yanukovich. Russia subsequently annexed Ukraine's Crimean peninsula.
Poroshenko is gearing up for a diplomatic push to sell his plan but his biggest challenge will be to win over Russian President Vladimir Putin. Relations with Moscow are at rock bottom and Kiev accuses Moscow of fomenting the unrest.
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.
The Kremlin on Friday denounced the ceasefire as an ultimatum rather than a peace offering and Russian Foreign Minister Sergei Lavrov expressed concern about Ukrainian military action.
"It is disturbing and raises concerns that, simultaneously with this (ceasefire) announcement...the so-called anti-terrorist operation is increasing," Interfax quoted him as saying during a visit to Saudi Arabia.
RUSSIAN MILITARY MOVEMENTS
Ukraine, for its part, 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 an apparent sign of defiance to Porosheko's peace plan.
In a ceremony on the town's Lenin Square, armed fighters, some wearing face 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 one 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 fearing 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 Jon Boyle)
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