"We have freed two Americans that we arrested earlier. One of the two is a CIA agent, the other was enlisted," Zakharchenko told journalists, without giving any more details.
The two men are believed to have been working for the International Rescue Committee (IRC) NGO in war-torn east Ukraine.
IRC head David Miliband released a statement saying that two of the organisation's workers who had been detained by the rebels some 10 days ago were now in a safe location, the BBC reported.
Over 30 IRC workers were reportedly detained after separatists raided their offices in Donetsk.
More From This Section
At the end of April several representatives of the New York-based NGO were expelled from Donetsk over espionage allegations after the rebels claimed to have found listening devices in their office.
"This is the first diplomatic scandal for our republic. All the devices were seized and handed over to the American authorities," Zakharchenko said.
A spokesman for the US embassy in Kiev said that they did not have any information on the release of the Americans.