Russian security authorities on Tuesday prevented the sister of a jailed Ukrainian pilot who stands accused of murdering two Moscow reporters from testifying in her defence.

Ukrainian pilot Nadiya Savchenko faces up to 20 years in jail in a controversial case that has seen Moscow condemned by human rights defenders as well as by the Kiev authorities.

Her sister Vira was meant to appear as the main witness for the defence in the ongoing trial.

But she wrote on Facebook that Russian authorities had told her at the border that she had been banned from entering the country until 2020.

"A guard on the Russian border informed me that I am banned from entering the Russian Federation until 2020," Vira Savchenko wrote.

She said the border guard said the orders had been issued by the powerful Federal Security Service (FSB) -- Russia's equivalent to the US Federal Bureau of Investigation (FBI).

A spokesman for the FSB in Moscow refused to when contacted by AFP.

The 34-year-old helicopter pilot has denied accusations that she helped direct an artillery strike that killed two Russian state television reporters in eastern Ukraine in June 2014.

Her lawyer Mark Feygin tweeted that "Nadiya Savchenko has now been deprived of her right to a defence."

The 18-month conflict has killed more than 8,000 people and badly damaged Moscow's relations with the West.

Source: AFP