An undated photo made shows former Vietnamese oil executive Trinh Xuan Thanh. (Vietnam News Agency/European Pressphoto Agency)

BERLIN — For months, Vietnamese agents had followed the trail of a former oil executive who allegedly looted a fortune from the state-run company. They finally found him in a two-square-mile forested park in the center of Berlin last month and spirited him back to Vietnam.

German authorities were stunned at what they called a state-run abduction, and now are demanding that 51-year-old Trinh Xuan Thanh be returned to complete his bid for asylum in Germany.

It has now become a struggle over whose justice should prevail — Vietnam's effort to have Thanh face charges that could carry a death sentence versus the pan-national codes of asylum — with consequences for trade and E.U. relations with Vietnam, whose economy is rapidly expanding.

Berlin has called the kidnapping an “unprecedented” violation of German and international law, which requires that the foreign national’s asylum claim, and the government’s extradition request, be processed in Germany.

Thanh is accused of causing about $150 million in losses at PetroVietnam, the state oil company where he had been an executive. Embezzlement charges carry a potential death sentence in Vietnam. The accused was seeking asylum in Germany.

Markus Ederer, state secretary at Germany's Foreign Ministry, is demanding that Thanh be allowed to return to Germany to complete the asylum process.

In the meantime, German authorities have escalated countermeasures, declaring Vietnam's intelligence attache “persona non grata” and giving him 48 hours to leave the country.

“There is no serious doubt about the participation of the Vietnamese intelligence service and embassy in the kidnapping of a Vietnamese citizen on German soil,” a spokesman for the Foreign Ministry told reporters.

Thanh’s attorney, Petra Schlagenhauf, said in a statement that her client was forcibly abducted in broad daylight on July 23 near the Tiergarten park. He was due to appear the next day for a hearing on his asylum request.

Germany’s Foreign Ministry said Vietnamese officials made an appeal for Thanh’s extradition at the Group of 20 economic summit last month in Hamburg, where Prime Minister Nguyen Xuan Phuc participated in talks with German Chancellor Angela Merkel.

The government in Hanoi, meanwhile, claims Thanh turned himself in after a nearly year-long international manhunt. An arrest warrant for Thanh was issued in September.

On Thursday, Vietnamese Foreign Ministry spokeswoman Le Thi Thu Hang called Germany’s comments “very regrettable” and insisted that Thanh surrendered to police in Vietnam last Monday.

“Vietnam always attaches importance to and wishes to develop strategic relations with Germany,” she told reporters.

Hours later, Vietnamese state television read a purported statement from Thanh saying he returned to Vietnam and "presented myself at the investigative authority" after living a "precarious and anxious life" in Germany, the Reuters news agency reported. It was not possible to independently confirm the authenticity of the statement.

Thanh served as chairman of a PetroVietnam subsidiary until 2013 and later took on several high-level positions in the government. In May 2016, he gained a seat in Vietnam's National Assembly but was expelled — and booted from the Communist Party — amid the investigation at PetroVietnam.

Germany is Vietnam’s largest European Union trading partner. The bloc is set to weigh a free trade deal with Vietnam, the ninth-most populous Asian country. Talks over the agreement began in 2012.

Brian Murphy in Washington contributed to this report.