President Ali Abdullah Saleh was heading to the United States on Saturday, Yemen's state news agency said, a week after leaving for Oman under a plan for him to step down to end a year of protests against his rule.
In London, a foreign office official said Saleh's plane had stopped at a British airport en route to the United States.
The state news agency said he had left the Omani capital Muscat, his home for the past week, to receive medical treatment.
Saleh has transferred some powers to his deputy and enjoys immunity from prosecution under a deal meant to end increasing instability in Yemen.
The deal also established a transitional government including the opposition and envisions restructuring Yemen's armed forces, key units of which are led by Saleh's relatives.
The United States, which endorsed the plan to coax Saleh out of office by granting him immunity from prosecution over the deaths of protesters, had defended its decision to issue him a visa, despite criticism that it would be seen as sheltering him.
Saleh had originally been expected to stop only briefly in Oman after leaving the Yemeni capital Sanaa last Sunday. He had said in a parting speech he would return to Yemen.
A foreign diplomat in Oman said, however, that Saleh had sought permission to reside there. An Omani government source declined to confirm or deny receiving such a request, but said Oman would be reluctant to agree to it in case this harmed future ties with Yemen.
The United States and Saudi Arabia fear protracted political upheaval in Yemen could give al Qaeda's regional wing a foothold near oil shipping routes through the Red Sea.
Underlining the continued violence, Yemeni troops killed four Islamist fighters in the southern town of Zinjibar, a local official said on Saturday.
He said the four fighters from Ansar al-Shariah were killed in a skirmish on the eastern edge of Zinjibar, which Islamists fighters overran last May. One soldier was killed in the fighting late on Friday, he said.
A spokesman for the Islamist group said his side had suffered no casualties, but confirmed the account of a soldier's death.
Saleh's foes accuse him of deliberately ceding territory to Islamists to make himself indispensable to his former U.S. patrons, and of ultimately aiming to sabotage the political transition and retain power for his inner circle.
(Additional reporting by Mohammed Ghobari and Tom Finn in Sanaa; Writing by Joseph Logan; Editing by Myra MacDonald)