Germany on Wednesday accused Vietnam of kidnapping a Vietnamese asylum seeker in Berlin and ordered a Vietnamese intelligence officer to leave the country within 48 hours.
Bui Thanh Hieu, a Vietnamese blogger based in Bangkok, said had recently spoken to Thanh.
"The kidnapping of Vietnamese national Trinh Xuan Thanh on German soil is an unprecedented and scandalous violation of German and worldwide law", said Mr Schaefer, warning it could damage relations between the two countries.
Local media reported that 51-year-old Trinh Xuan Thanh was kidnapped on July 23 in Berlin's Tiergarten by several armed men and secretly transported to Vietnam.
"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 foreign ministry spokesman told reporters yesterday.
He said the kidnapping "has the potential to negatively affect relations massively".
"Vietnam." As a first step, the official representative of Vietnam's intelligence agency in Germany was to be "declared persona non grata" and given 48 hours to leave the country.
Trinh Xuan Thanh was accused of causing about $150m (£113m) in losses at a Vietnamese state firm.
The 51-year-old Thanh was former head of the state-owned Petro Vietnam Construction Corporation, and was accused of causing about 150 million US dollars in losses.
Vietnam's ambassador was summoned to the foreign ministry on Tuesday and was told that Germany demanded that Thanh be returned so that the asylum and extradition proceedings could be conducted properly. He was elected to Vietnam's National Assembly in May 2016, but was dismissed a short time later after being accused of corruption.
That prompted Communist Party head Nguyen Phu Trong to order an investigation into his career and how he had been given further promotions despite the alleged losses at PetroVietnam Construction. Dozens of high-ranking Vietnamese government and party officials have been arrested in the past few months - with some sentenced to death.