Two Ukrainian Su-25 fighter jets were shot down in the disputed region of Donetsk by missiles suspected to have been fired from inside Russia.
The Ukrainian authorities have claimed that the missiles that hit the jets came from across the border, while the US intelligence agencies suspect Russia to have supplied additional weaponry to the rebels after the crash of the MH17 last week.
The Ukraine National Security Council said that the fighter jets were shot down by Russia at an altitude of 5,200m on Wednesday.
"According to preliminary information, the missiles were launched from the territory of the Russian Federation," the council said in a statement.
The jets were shot down in the south of the Lugansk region close to the village of Dmytrivka, around 25km from where the Malaysia Airlines MH17 had crashed.
Ukraine spokesman Oleksiy Dmytrashkivsky said: "Two Sukhoi Ukrainian fighter jets have been shot down. The fate of the pilots is not known."
According to reports, the two pilots have ejected from the jets but their whereabouts are not yet known.
In a statement on its website, the self-proclaimed Donetsk People’s Republic of pro-Russia separatists claimed to have downed the jets.
One of the pilots was killed, while the other was being sought by rebel fighters, the separatists said in a statement seen by Reuters.
The MH17 crashed on 17 July in eastern Ukraine, killing all 298 passengers onboard.
Ukraine had alleged that the rebel fighters shot down the Boeing 777, which was on its way to Kuala Lumpur from Amsterdam, using a surface-to-air missile.
However, the insurgents have denied the allegations claiming that they do not have the required arms power.
Pro-Russia rebels are reported to have brought down several Ukrainian military aircraft, including an Antonov An-26 transport plane and a Sukhoi Su-25 fighter during their 15-week insurgency in the eastern region of the country.
Image: A map released by the Ukraine National Security Council on the prevailing situation in the country. Photo: National Security and Defense Council of Ukraine.