"Prime Minister Koirala left Saturday afternoon for medical check ups in Bangkok," said Basanta Kumar Gautam, the premier's assistant.

The 84-year-old prime minister is reported to have serious problems with his lungs and recently ended a lifelong smoking habit.

He was accompanied by three Nepalese doctors and was expected to remain in the Thai capital for a week, his aide said.

Friday saw a historic meeting between the prime minister and rebel Maoist leader Prachanda that resulted in an agreement to dissolve the recently reinstated government and form an interim government, containing the rebels.

The elder statesman of Nepalese politics, Koirala has served as prime minister numerous times in the past. He assumed the top job in April again after weeks of protests forced King Gyanendra to end 14 months of direct rule.

Since coming back into power, parliament has stripped the king of his former political role and removed his control of the army.

Since the rebels began their "people's war" in 1996, some 12,500 people have died.