Germany's top human rights official said Wednesday he had been refused permission for a visit to Belarus to assess the situation there and visit a rights activist in prison.

Markus Loening said in a statement he had scheduled a three-day visit to Minsk to hand over a letter on humanitarian concerns from Foreign Minister Guido Westerwelle to Belarus President Alexander Lukashenko.

"The Belarussian government has refused the visit," Loening's office said.

"I regret I am not welcome in Minsk. My visit was supposed to allow me to get a picture of the human rights situation on the ground."

The struggling ex-Soviet republic has turned into one of the world's most isolated states under the nearly 18-year leadership of Lukashenko, who was once dubbed the last dictator of Europe by the United States.

Belarus currently faces sanctions and travel restrictions from both the EU and the United States.

In November, authorities there jailed Vyasna (Spring) organisation leader Ales Beliatsky for four and a half years on tax evasion charges.

Loening had also planned to visit Beliatsky during his abortive trip.