President Ali Abdullah Saleh has left the Omani capital Muscat after a brief layover and is en route to the United States for medical treatment, Yemen's official news agency reported on Monday.

The announcement came a day after Saleh pleaded for forgiveness from his compatriots for "any shortcomings" during his 33-year-rule, in a televised farewell speech which appeared to mark his official departure from power.

"The president...is on his way to the United States to continue what is left of his medical treatment" for wounds sustained during a June bomb attack on his compound, the official SABA news agency said in a statement on its website.

Saleh left Yemen Sunday evening for neighbouring Oman with his five youngest children and his wife, according to a source close to the president.

In his speech on Sunday, Saleh said he would return to Yemen but not as president, the strongest sign yet that the veteran leader intends to follow through with the conditions of a Gulf-brokered transition plan which calls for his ouster.

"I will go to the United States for treatment and will then return as head of the GPC party," he said referring to his General People's Congress party.

For months, Saleh refused to sign the power-transition plan despite near daily mass protests calling for his resignation and regional and international pressure demanding he step down.

The deadlock threw Yemen into chaos and left the economy of this already impoverished nation in shambles.

The United States on Sunday approved a visit from Saleh for medical treatment but said it was with the understanding that he would stay only for a "limited time."

"Ali Abdullah Saleh's request to travel to the US for medical treatment has been approved," the US State Department said.

"As we have indicated, the sole purpose of this travel is for medical treatment and we expect that he will stay for a limited time that corresponds to the duration of this treatment."