However, the differences will be sorted out soon, after holding a dialogue with the UML leaders, a senior leader of the Nepali Congress said.
Later, Koirala went to his new office at Singha Durbar and met senior officials.
Koirala said that his priorities are peace, stability, democracy and development. He said he would make every effort to promulgate a new statute within a year.
“The new constitution may be issued earlier also,” Koirala said.
Koirala, who is unmarried, has so far not taken up any administrative post and has no experience of handling a ministry in the past.
He served six years in prison on different occasions in Nepal and India.
Koirala, the country's sixth Prime Minister since monarchy was abolished in May 2008, went to India in 1960 after the then King suspended democracy and jailed dozens, including his relatives.
He stayed there for 16 years in self-exile.
