Yemen's outgoing President Ali Abdullah Saleh is seeking exile in neighbouring Oman, but the sultanate is reluctant to host him for fear of hurting its relations with any future Yemeni government, diplomats said on Tuesday.

Saleh left Sanaa on Sunday and headed to the United States for medical treatment following a brief stopover in Oman, though he said in a parting speech he would return to Yemen.

A foreign diplomat in Muscat said Saleh has 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 grant it in case this might harm future relations with Yemen.

The United States, which endorsed a plan to coax Saleh out of office by granting him immunity from prosecution over the deaths of protesters during an uprising against his rule, defended its decision to issue him a visa, despite criticism that it would be seen as sheltering him.

We ... believe that his absence from Yemen at this critical juncture will help facilitate a transition that completes the end of his rule, helps Yemen and ultimately has a positive effect on the rights and dignity of the Yemeni people, White House spokesman Jay Carney said on Monday.

Our policy focus remains on preventing further instability and keeping that transition on track, he said, adding that Saleh would stay in the United States for a limited time only.

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.

Despite Saleh's departure, many believe he and his supporters will still wield influence over Yemen, which has seen a year of anti-government demonstrations punctuated by warfare between Saleh's forces, those of a rebel general, and tribal militias.

Yemeni air force officers blocked main roads in the capital on the third day of a strike demanding the resignation of their commander, a half-brother of outgoing President Saleh, witnesses said.

Hundreds gathered outside the residence of Yemen's acting leader Abd-Rabbu Mansour Hadi, calling for General Mohammed Saleh al-Ahmar to be dismissed while others sat in the road, preventing cars from circulating.

The strike is part of a wave that has gripped Yemen over the past month, after Saleh signed a Gulf-brokered deal formally handing power to his deputy Hadi in November.

Political turmoil has deepened an existing humanitarian crisis in Yemen, which has half a million people displaced by various internal conflicts, including fighting between government troops and Islamists in the south of the country.

UNICEF's director for Middle East and North Africa Maria Calivis told a news conference in Sanaa on Tuesday that 500,000 Yemeni children are now at risk of death due to malnourishment.

(Reporting by Saleh al-Shaibany in Muscat and Mohammed Ghobari in Sanaa; writing by Isabel Coles; editing by David Stamp)