NAIROBI, Kenya — In a highly contentious move, Somalia’s president has extended his own term in office by two years, drawing condemnation from the United States and other allies who viewed the move as a naked power grab and feared it could upend faltering efforts to establish a functioning state and defeat the insurgency by the extremist group Al Shabab.

President Mohamed Abdullahi Mohamed, a one-time American citizen popularly known as Farmaajo, announced that he signed the law extending his mandate early Wednesday, two days after it was approved by a majority of Somalia’s Parliament amid accusations that the president’s office had engineered the vote.

The move represented a worst-case scenario for United Nations and Western officials, who had been shuttling for months between Mr. Mohamed and Somali regional leaders locked in a bitter dispute over when and how to hold parliamentary and presidential elections that were scheduled to take place by early February.

The United States, which has given billions of dollars in aid to Somalia and conducted numerous airstrikes and military raids against Al Shabab, had privately threatened Mr. Mohamed and his top officials with sanctions and visa restrictions if he disregarded the election time table.