Story highlights State Department spokeswoman condemned what she called a "kidnapping"

Anton Skiba was seized Tuesday in Donetsk

Skiba was taken to a waiting car by gunmen after working at the MH17 crash site

CNN chose not to report it while working for his release

A Ukrainian journalist working as a freelancer for CNN remains in detention two days after he was detained by pro-Russian separatists.

Armed men from the self-declared Donetsk People's Republic seized Anton Skiba outside a hotel in the separatist-controlled city of Donetsk after he had worked for one day with a CNN television crew.

Since his detention, CNN has attempted through a number of different separatist officials, including the office of the separatist Prime Minister Alexander Borodai, to secure Skiba's freedom.

CNN chose not to report his detention 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.

Anton Skiba was working for CNN when he was abducted in Ukraine on Tuesday night.

Armed fighters led by a senior official from the rebel region were waiting outside the Donbass Palace Hotel when CNN's crew returned from a day's work at the MH17 crash site on Tuesday evening.

The official, who later introduced himself as Alexandr Kalyussky, the deputy prime minister in charge of industry in the Donetsk People's Republic, initially accused Skiba of "terrorism" and of posting cash rewards for the killing of separatist fighters on his Facebook page. Kalyussky showed a folder of printed documents that he claimed was evidence, including an apparent printout of Skiba's Facebook page.

Gunmen escorted Skiba to a waiting car. The young man did not resist as he was led away.

Kalyussky seized a cell phone from CNN cameraman Jeff Kehl, who tried to film the detention. Later, Kalyussky apologized and returned the cell phone to Kehl on condition that Kehl delete the video.

In subsequent conversations with CNN, Kalyussky dropped the accusation that Skiba was offering cash rewards for assassinations. Late Tuesday night, Kalyussky said Skiba was being questioned for having multiple forms of identification with different surnames. On Wednesday, another high-ranking separatist official told CNN that Skiba admitted to being a "Ukrainian agent."

The U.S. ambassador to Ukraine said he was "very alarmed" by the incident. Geoffrey Pyatt, the envoy, laid the blame at Moscow's door.

He told CNN's Christiane Amanpour the Kremlin was trying "to use information and fear as part of its strategy to sow chaos in Ukraine. That's a strategy that leads to a dead end for Russia."

State Department spokeswoman Marie Harf strongly condemned what she called Skiba's "kidnapping" by Russian-backed separatists.

"We demand his immediate release along with the other hostage I believe they hold," she said.

Several international human rights and press freedom organizations have called for Skiba's immediate release.

"I strongly condemn this incident. Journalists must not become targets just for fulfilling their professional duties," said Dunja Mijatovic, the representative for media freedom at the Organization for Security and Co-operation in Europe.

The Committee to Protect Journalists, Reporters Without Borders, and the United Nations Human Rights Monitoring Mission in Ukraine have also expressed concern about Skiba and urged his release.

"The climate for press freedom in the volatile eastern Ukraine has deteriorated further," the CPJ warned. The group reported that separatists have detained up to 10 foreign correspondents in the last week who came to the region to report on the plane crash.

On Tuesday, a British reporter working for the television network Russia Today and a cameraman for the Abkhazian Network News Agency went missing while reporting on clashes on the outskirts of Donetsk, the CPJ said.

RT freelancer Graham Phillips and an ANNA cameraman identified only as Vadim have been missing since Tuesday, the CPJ said. They were last seen near Donetsk airport.

On Wednesday, Skiba made a short phone call to CNN.

He said he was being questioned at the headquarters of the Donetsk security services and added that he would "not be available" before the call was abruptly cut off. It was unclear whether the call was made under duress.

On Thursday, separatist officials did not respond to additional requests from CNN about Skiba's welfare.

On July 11, Kalyussky -- the separatist official who detained Skiba -- was added to the growing list of DPR officials to face sanctions from the European Union.

Anton Skiba had previously also worked as a fixer for the BBC for two days in the wake of the Malaysia Airlines Flight 17 crash. A journalist at the weekly Moscow-based newsmagazine Russian Reporter told CNN that Skiba also worked for several months in 2013 as a photographer for the publication.

He was working as a fixer for CNN -- a freelance position that combines journalism, translating and providing local knowledge.