Three members of a German family, including a United Nations expert, are being held in a mountainous area of Yemen after being kidnapped, a security official has said.

The three were seized on Sunday in the Bayda area southeast of Sanaa and taken to Bani Dhabian, 60km east of the capital, the official said on Monday.

The kidnappers are believed to be tribesmen who have frequently used foreign hostages as bargaining chips with the government.

A Western diplomat said the hostages were a German couple and their daughter, who works in Yemen.

"The mother and father were on a visit in Yemen, and they were kidnapped with their daughter as they were sightseeing in the province of Bayda," the diplomat said.

Jens Ploetner, a German foreign ministry spokesman, said the three had been missing since Sunday from the outskirts of Sanaa and "we assume they have been kidnapped".

Yemen's powerful tribes have abducted more than 200 foreigners over the past 15 years to put pressure on the government in Sanaa to respond to their demands.

All have been freed unharmed except for three Britons and an Australian seized by militants in December 1998 who were killed when security forces stormed the kidnappers' hideout.