A spokesman for Ukraine's security council says an air force fighter jet has been shot down by a missile fired from a Russian plane.

Ukrainian officials have recently accused Russia's armed forces of being directly involved in attacks on Ukrainian troops battling an insurgency near the Russian border.

A Ukrainian spokesman, Andrei Lysenko, said in a televised briefing that the pilot of the Sukhoi Su-25 jet was forced to eject from the plane after it was shot at on Wednesday evening. It was the first time Ukraine has directly accused Russia of using air power in the fighting. Russia's defence ministry denied responsibility for downing the plane, calling the accusation "absurd".

The Ukrainian defence ministry said the plane crashed on Wednesday night near Amvrosiyivka, about nine miles (15km) from the border with Russia, after rockets hit the tail as the aircraft wheeled away from the border.

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

It was the third reported shooting down of a Ukrainian plane with a missile this week.

Kiev said an An-26 military transporter was brought down last Monday probably by a missile fired from Russia, either from the air or the ground. Two of the eight people on board 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 defence ministry said the second jet was hit by a portable surface-to-air missile, but that the pilot was unscathed and managed to land his plane safely.

On Monday, Ukraine said one of its military transport planes carrying eight people was shot down by a missile fired from Russian territory. The Ukrainian security service chief, Valentyn Nalyvaichenko, said he had "unconditional evidence" that Russia was involved in downing the craft.

The downing of the Su-25 comes 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 it is supporting the separatists or sowing unrest within its neighbouring state.