Vietnamese former state oil executive Trinh Xuan Thanh withdrew his appeal and failed to appear in court for "health reasons" on Monday, according to the Communist Party of Vietnam website

A Vietnamese oil executive allegedly kidnapped from Germany withdrew his appeal against a life sentence for corruption Monday, as Slovakia distanced itself from reports its plane was used to brazenly abduct him.

Hanoi denies the Cold War-style kidnapping of Trinh Xuan Thanh from a Berlin park in July 2017, where he was reportedly bundled into a van and spirited back to Vietnam to face corruption charges. It claims he returned voluntarily.

The case has captivated Vietnam, where a hardline administration is waging a massive anti-graft sweep against once-powerful figures that critics say is politically motivated.

Thanh was spared the death sentence and given life in prison for embezzlement in January.

He withdrew his appeal and failed to appear in court for "health reasons" on Monday, according to the Communist Party of Vietnam website.

His family also withdrew its appeal for the state to return its confiscated properties, the report said without providing details.

The case has badly bruised ties between Hanoi and Berlin, which has expelled Vietnamese diplomats over what it calls a "scandalous violation" of its sovereignty.

Slovakia is the latest country to be drawn into the tangled affair.

German newspapers have reported that Slovakia lent a government plane to a Vietnamese delegation visiting Bratislava days after the kidnapping for two flights, from Prague to Bratislava and then on to Moscow.

The reports claim Thanh was transported on the first flight to Bratislava.

Slovakia said there were four Vietnamese officials on the flight to Bratislava, but no one appeared under duress.

"The entire Vietnamese delegation underwent a thorough police check, all of them had diplomatic passports, no one boarded the aircraft handcuffed, no one was forced onto the plane under pressure," Slovak Interior Minister Denisa Sakova said in a televised debate Sunday.

Slovakia earlier warned Hanoi of harsh consequences if it had been duped into transporting Thanh as suggested by the German reports.

"We will consider this a manifestation of gross incorrectness by the Vietnamese partner, an unfriendly abuse of our hospitality and a deterioration of the well-developed bilateral relations," it said.

German Chancellor Angela Merkel said at a meeting in Berlin last week with Slovakia's Prime Minister Peter Pellegrini that she had been promised by Bratislava that the matter would be cleared up, adding that all the facts "must be on the table".

A Vietnamese-Czech dual citizen has been held and is currently on trial in Germany.

The man, identified as Long N.H., allegedly rented vehicles used in the kidnapping. His lawyers say he is a "sacrificial pawn" who had been told by agents that he was renting the vehicles for tourism purposes.

Thanh is the former head of a construction unit at the country's biggest oil firm PetroVietnam.

He was accused along with 21 others of causing losses worth $52 million for the state over the construction of a thermal power plant.

He was handed a second life sentence in a separate corruption trial this year.