(Reuters) - Rebels shot down a Ukrainian helicopter carrying technicians who had been installing equipment to monitor violations of a peace plan in Ukraine's rebellious east on Tuesday, killing all nine people on board, a military spokesman said.

The technicians had been returning from setting up specialised equipment when their Mi-8 cargo helicopter was struck by a rebel missile near Slaviansk in eastern Ukraine, government forces spokesman Vladyslav Seleznyov said.

"There were 9 people on board. According to preliminary information ... all those on board were killed," Seleznyov said on his Facebook page. The nine dead included a three-man crew.

"The (rebel) fighters, having fired the rocket, hid in the nearby village of Bylbasovka," he said.

The incident took place just hours after pro-Russian separatists on Monday night announced a ceasefire until June 27 to match a week-long truce by government forces which has been ordered by President Petro Poroshenko.

It was the second time a helicopter has been brought down by rebel fire from Slaviansk, a separatist stronghold. On May 30, rebels there also downed a military helicopter killing 14 servicemen, including one general.

Dmytro Tymchuk, a military analyst known to have good sources in the armed forces, said the helicopter had been brought down by a missile fired from a shoulder-held launcher.

The truce announcement on Monday night by separatist leaders had raised the first real prospect of an end to hostilities since the insurgency erupted in the largely Russian-speaking east in April.

But Tuesday's helicopter incident seemed certain to put the ceasefire on both sides under fresh pressure.

Elsewhere, a rebel spokesman in Donetsk said a shootout broke out near the city's airport, which is under Ukrainian control. Rebels opened fire on two Ukrainian armoured personnel carriers leaving the grounds of the airport, according to a witness.

SAFETY CORRIDOR

Earlier on Tuesday Poroshenko, who is trying to secure national and international support for his plan to end two and a half months of fighting, had said the pro-Russian separatists had violated their own ceasefire with overnight attacks on military posts and checkpoints which killed one government soldier and wounded seven others.

His plan would offer a safety "corridor" back to Russia for pro-Russian fighters who lay down their arms and has secured the backing of Western governments and qualified support from Russian President Vladimir Putin.

About 150 Ukrainian military personnel have been killed as well as scores of other law-enforcement officials, rebel militia and civilians since pro-Russian separatist groups declared "people's republics" in the east and said they want to join Russia. Some of the rebels say they will adhere to the ceasefire announced on Monday night after talks between separatist leaders and a "contact" group of officials who included a former Ukrainian president, Moscow's envoy to Kiev and a high-ranking representative from the OSCE security and rights watchdog.

But in the rebel stronghold of Slaviansk, near where the helicopter was brought down on Tuesday, separatists appeared to be dissociating themselves from the ceasefire agreement.

"Talks with them (the Kiev government) are possible only from a position of force and no other way. We should not trust a single letter," said Igor Strelkov, the top rebel commander in Slaviansk, as quoted on the Facebook page of Pavel Gubarev, the self-styled governor of the "Donetsk People's Republic".

"They (Ukrainian troops) are trying to quickly and calmly take and reinforce new positions under the cover of the ceasefire treaty," Strelkov was quoted as saying.

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