Ukrainian Air Force pilot Nadiya Savchenko and film director Oleh Sentsov, who are in Russian custody, may be exchanged for pro-Russian rebels captured by Ukrainian forces in the conflict in eastern Ukraine.
The chief of Ukraine's Security Service, Valentyn Nalyvaychenko, told journalists on September 10 that Savchenko and Sentsov are on a list of people to be exchanged.
A cease-fire deal agreed by Kyiv and the rebels on September 5 calls for the swift release of all "hostages and people held illegally."
There was no immediate comment from Russia, which has charged Savchenko with complicity in the deaths of two Russian journalists in eastern Ukraine.
Sentsov has been charged with planning terrorist attacks in Crimea after it was annexed from Ukraine by Russia in March.
Ukrainian President Petro Poroshenko said on September 10 that some 700 Ukrainian citizens had been freed from rebel captivity and that 500 others would be released soon.
Rebels in the Donetsk region said the exchange of more captives scheduled for September 10 may be postponed by a day.