A pro-Russian separatist from the Vostok (East) battalion poses for a picture atop a T-64 tank in Donetsk, eastern Ukraine. Picture: Maxim Zmeyev

Kiev - A Russian jet 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, a Ukrainian military spokesman said on Thursday.

It was the strongest Ukrainian allegation to date of direct Russian military involvement in the conflict. Russia's defence ministry declined to comment on the report.

The plane was brought down on Wednesday night by a rocket strike and the pilot safely ejected, Andriy Lysenko, a spokesman for Ukraine's national Defence and Security Council, told journalists.

It 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 brought the plane down successfully with relatively slight damage. Kiev did not allege Russian involvement in that case.

The incidents come against a background of increasingly strident charges by Kiev of direct Russian involvement in the three and a half month conflict.

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

After a day's pause, Ukrainian warplanes on Wednesday resumed overflights in the east where they have been striking at concentrations of rebels and military equipment which Kiev says is being brought in from Russia to fortify rebel positions.

Lysenko said five more Ukrainian servicemen had been killed in the past 24 hours, which 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. - Reuters