By Richard Balmforth

KIEV (Reuters) - Russian jets shot down a Ukrainian SU-25 fighter plane that was on military operations over the east of Ukraine, where government forces are fighting to quell a pro-Russian separatist rebellion, the Ukrainian military said on Thursday.

It was the first time Ukraine had directly accused Russia of using air power in the war. In a previous attack on a military transporter, which it said was launched from Russia, Kiev was unable to specify whether it came from landbased missiles or airborne.

Russia's defence ministry declined to comment on Thursday's accusation by Kiev.

The Ukrainian Defence Ministry said the plane was brought down on Wednesday night near Amvrosiyivka, about 15 km (about 9 milles) from the border with Russia, by rockets which hit it in the tail as it wheeled away from the border.

"It is likely that this was carried out by air-to-air rockets from the Russian airforce which were patrolling the border in a pair," the ministry said in a statement on its website.

The pilot safely ejected, Andriy Lysenko, a spokesman for the National Defence and Security Council, told journalists.

The downing of the SU-25 came against a background of increasingly strident charges of direct Russian involvement in the three and a half month conflict in which the pro-Western government in Kiev is fighting to put down a rebellion by separatists who want a future in Russia.

Moscow denies orchestrating the rebellion. But Western governments accuse it of failing to do enough to help curb the violence. U.S. President Barack Obama imposed sanctions on Wednesday on some of Russia's biggest companies, limiting their access to funding.

"VERY TOUGH"

"The situation on the border in the zone of the 'anti-terrorist operation' is still very tough. Grad missile systems, heavy artillery and mortar is continually being used. The firing on the border posts and (government) forces is often coming from the territory of the Russian Federation," Lysenko said.

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Five Ukrainian servicemen had been killed in the past 24 hours, he said. This would bring to more than 270 the number killed since the government launched an "anti-terrorist" operation in April to crush the rebels.

Hundreds of civilians and rebels have also been killed.

Ukrainian positions had come under fire from artillery from the Russian border settlement of Kuybyshevo, Lysenko said, adding that more and more Russian units were coming up to the border with Ukraine.

A Ukrainian paratroop tactical group deployed at Dmytrivka in particular had come under heavy fire from the Russian side, he said.

In the past 24 hours, the separatists had carried out 27 attacks on army checkpoints and positions of government forces, Lysenko said.

Attack planes are one of the Ukrainian military's most effective weapons for inflicting heavy losses on concentrations of rebels and military equipment which Kiev says is being brought in from Russia to fortify rebel positions.

The shooting down of the SU-25 was the third reported incident this week in which a Ukrainian plane has been hit by a missile.

Kiev has said that an An-26 military transporter was brought down last Monday probably by a missile fired from Russia, either from the air or from the ground. Two out of the eight people on board that plane were killed, the Ukrainian military said.

On Wednesday, another SU-25 was hit by a rebel missile but the pilot landed the plane successfully with relatively slight damage. Kiev did not allege Russian involvement in that case.

The rebellions erupted in Ukraine's Russian-speaking eastern regions after months of pro-Europe protests drove out a Moscow-backed president. Russia subsequently annexed Ukraine's Crimea, sparking the biggest Russia-West crisis since the end of the Cold War.

Ukrainian President Petro Poroshenko refused to renew a 10-day unilateral ceasefire by government forces on June 30, saying it had been repeatedly breached by the separatists and had cost Ukrainian lives. Efforts to forge another more effective truce have failed.

(Additional reporting by Natalya Zinets in Kiev and Tatyana Ustinova in Moscow; Writing by Richard Balmforth)