BRATISLAVA (Reuters) - Slovakia demanded on Thursday that Vietnam explain media reports that Vietnamese agents used a Slovak government plane to smuggle home an ex-oil executive they kidnapped from Berlin last year.

The German newspaper Frankfurter Allgemeine Zeitung reported last week that Vietnam may have used a Slovak plane to transport Trinh Xuan Thanh, who Germany said was kidnapped in a Berlin park last July.

Thanh, who had sought political asylum in Germany, was sentenced to life in prison in January for violating state regulations and embezzlement.

The Slovak Foreign Ministry said on Thursday it had summoned the Vietnamese ambassador to relay its questions and concern over the case.

“If these allegations are confirmed, we will see it as a serious incident with a negative impact on our bilateral relations,” the ministry said in a statement. “We strongly refuse that Slovakia had anything to do with such act ... “

It said it may take further diplomatic steps if Vietnam does not provide a credible explanation.

The Vietnamese government did not immediately respond to a Reuters request for comment outside office hours.

On Wednesday, Slovakia’s Prime Minister Peter Pellegrini distanced the government from the report that Slovakia helped Vietnam with the alleged kidnapping and also said there was no evidence that Thanh was on the plane.

“The only possible explanation would be an abuse of Slovakia’s hospitality, but nothing suggests that was the case so far,” Pellegrini told reporters before leaving for a meeting with German Chancellor Angela Merkel.

Slovakia lent a government plane to a Vietnamese delegation last year to fly to Moscow from Bratislava after a meeting between then-interior minister Robert Kalinak and Vietnam’s minister of public security, To Lam.

Kalinak told Slovak news website www.aktuality.sk that the Vietnamese delegation was supposed to fly out of Vienna, just 45 km (28 miles) west of Bratislava, but requested the plane after its itinerary changed unexpectedly.

According to Frankfurter Allgemeine Zeitung, the car supposedly used for the abduction was parked in front of the Slovak government hotel where the Slovak-Vietnamese meeting was held three days after the abduction, according to GPS records.

Thanh, a former high flyer in Vietnam who was accused of mismanagement and causing losses at PetroVietnam Construction JSC, has been given two life sentences in Vietnam since he was brought home.

A Vietnamese man has been brought to trial in Germany for allegedly helping in the kidnapping.