The no-confidence motion was backed by lawmakers loyal to President Hassan Sheikh Mohamud after the two men fell out over a cabinet reshuffle last month.
Western donors who have promised to help rebuild Somalia’s battered institutions worry that the removal of a second prime minister in less than a year will weaken the government and leave it rudderless in its fight against Islamist rebels.
Washington warned Somalia late on Monday about the dangers of holding the vote, saying it was “deeply concerned with political turmoil”.
“Actions to put forward a parliamentary motion for a vote of no confidence in the prime minister do not serve the interests of the Somali people,” said Jen Psaki, a U.S. State Department spokeswoman.
Witnesses said security around the Somali capital was tightened on Tuesday, with increased police presence around the parliament building, which the al Shabaab Islamist militant group has targeted in the past.
“Today the speaker is going to distribute to lawmakers the no-confidence motion papers against the PM. Lawmakers will read and then debate,” legislator Dahir Amin Jesow told Reuters, adding that a vote was unlikely before the weekend.
A second lawmaker entering the parliament confirmed the debate would start on Tuesday.
The dispute erupted after Ahmed, an economist who has been in charge since December 2013, sidelined one of the president’s key allies in a cabinet re-shuffle.
The clash came at an awkward time for Somalia’s donors who have organized a major conference in Copenhagen later this month to lobby for more funds and support for the east African nation’s reconstruction.
Psaki said Washington no longer saw the use of sending a delegation to the Denmark meeting “because Somalia’s leadership is distracted with political division”.
The prime minister’s office decline to comment.
Ahmed’s predecessor was also sacked by parliament after a similar row with Mohamud that paralyzed the government for months last year.
Last week the United Nations and the European Union also warned that the row could inflame tensions and undermine the country’s recovery from more than two decades of conflict.
(Writing by Drazen Jorgic; Editing by Edith Honan and Andrew Heavens)