(CNN) -- Yemeni President Ali Abdullah Saleh left the United States on Thursday, flying from Boston's Logan International Airport to Addis Ababa, Ethiopia, a Yemeni official told CNN.
Saleh was in the United States for medical treatment for wounds suffered in a June assassination attempt at his presidential palace during battles between government troops and tribal fighters.
Saleh was expected to return to Yemen in time for the inauguration of the next president once a date for that is determined, the official said.
Voters in Yemen went to the polls Tuesday to replace Saleh, who led the country for 33 years. The only person on the ballot was Vice President Abdurabu Mansur Hadi, who became acting president in November as the result of a power transfer brokered by the Gulf Cooperation Council after months of protests.
U.S. Secretary of State Hillary Clinton called the election "another important step forward in their democratic transition process."
Human Rights Watch has called on Hadi to make changes without delay.
"Yemen's potentially historic transition will be off to shaky start unless Hadi makes an immediate break with the abuses of the past," said Letta Tayler, HRW's Yemen researcher. "Yemen's new leader needs to move decisively to usher in promised reforms that uphold human rights and the rule of law."
The 65-year-old Hadi is a British-, Egyptian- and Soviet-trained army officer, recently promoted to field marshal. He had been vice president since 1994 and ran for a two-year term as president on pledges of improving security and creating jobs.
But he's never had much of a power base, and Yemen's problems are expected to take longer than two years to fix. Yemen is the poorest country in the Middle East, with a severe shortage of water and rising levels of malnutrition among its population of about 25 million.
Saleh faced a separatist movement in the south, sectarian tensions in its north and the growing presence of what Western officials describe as al Qaeda's most dangerous affiliate, al Qaeda in the Arabian Peninsula.
CNN's Jamie Crawford contributed to this report