Anton Skiba was taken outside the Donbass Palace hotel in the rebel-controlled eastern city of Donetsk. He had worked for one day as a local fixer with a CNN television crew.
The network was in Ukraine covering the crash of Malaysia Airlines Flight MH17, which is believed to have been shot down in Eastern Ukraine last Thursday, killing 298 people.
CNN reported that as Skiba and the network's crew returned to the hotel from the crash site Tuesday evening, armed men approached Skiba, accused him of terrorism and calling for the death of pro-Russian separatists on Facebook, and then escorted him to a waiting car.
"Since his detention, CNN has attempted through a number of different separatist officials, including the office of the self-declared separatist Prime Minister Alexander Borodai, to secure Skiba's freedom," an article on the CNN website said.
"CNN chose not to report his abduction at the time while making efforts to obtain his release. That has not happened to date, so CNN is now publicly asking those who are holding Skiba to release him immediately."
CNN reported that Skiba's abductors included Alexandr Kalyussky, a deputy prime minister in the separatist-formed government that controls the rebel-held eastern region of Ukraine, which the separatists have dubbed the Donetsk People's Republic.
Kalyussky is one of the people named on the European Union's list of Russian and Ukrainian citizens against which it has imposed economic sanctions because of the ongoing conflict in the country.
Donetsk province is one of the eastern regions of Ukraine at the centre of several months of violent clashes between pro-Russia separatists and Ukrainian nationalists that support the central government that came into power after the ousting of Viktor Yanukovych in February. Donetsk is the administrative capital of the region and about 50 kilometres west of the MH17 crash site.
Several organizations have condemned Skiba's abduction and called for his release, including the Organization for Security and Co-operation in Europe (OSCE), which has been monitoring the situation in Ukraine, and the Committee to Protect Journalists.