Defense Secretary Robert Gates said Thursday the U.S. military was moving forward with plans to mount a relief mission in Myanmar, but he said he couldn't imagine air dropping aid without permission from the closed regime.

His comments followed those earlier Thursday by Ky Luu, the director of the U.S. Office of Foreign Disaster Assistance, that an air drop was one of the options being considered as Myanmar's junta continued to stall on accepting assistance from the United States.

Gates said the military was moving aircraft and ships into place to help deliver humanitarian supplies once permission is granted.

"I cannot image us going in without the permission of the Myanmar government," Gates said at a Pentagon press conference with Chairman of the Joint Chiefs of Staff Adm. Mike Mullen.

Asked if it would be helpful to victims for the U.S. to drop supplies, Mullen said: "We could. Typically, though, it's sovereign airspace and you'd need their permission to fly in that airspace."

"It's all tied to sovereignty, which we respect whether it's on the ground or in the air," Mullen said.

Luu told a State Department press conference earlier that air drops are often inefficient, could have broader international legal implications and that the best option would be for Myanmar, which is also known as Burma, to accept the aid.

Still, "anything that might have a positive impact is being looked at and is being discussed," he said, adding that air drops would be a last resort.

The comments came as the United States and other donor countries continued to wait for permission to enter with tons of assistance and disaster relief personnel to assess what the needs are and move toward distributing the aid.

Among other countries considering air drops are Italy and France, whose foreign minister has suggested the possibility of forcing assistance into Myanmar, officials said.

Pentagon officials have said they are wary of such a scenario because it could be considered an invasion. But French Foreign Minister Bernard Kouchner said this week that air drops could be allowed under the U.N.'s "responsibility to protect" mandate, which applies to civilians.

Officials said there were several problems with air drops into an unpermissive environment, especially if there are no experts on the ground to monitor the distribution of aid. Desperate people could riot over the assistance and there is the possibility that security forces might confiscate it and keep it out of the hands of the needy, they said.

The government has reported more than 20,000 deaths and more than 40,000 missing from Cyclone Nargis that hit Myanmar, particularly the Irrawaddy River delta, last weekend. A U.S. diplomat said Wednesday that the death toll in the delta could exceed 100,000. The U.N. estimates that a million people have been left homeless.

Meanwhile Thursday, the U.S. military stepped up preparations for any humanitarian mission to Myanmar, readying ships and Marines that were in the region for a multinational exercise.

The U.S. Air Force moved more airplanes to a staging area in Thailand and the Navy was transporting Marines and helicopters into Thailand from an aviation combat element of the USS Essex expeditionary strike group, officials said. Ships were to move later Thursday.

The Navy and Marine Corps happened to have ships and thousands of service members in the Gulf of Thailand for a multinational exercise on humanitarian missions — an exercise that started Thursday.

Because it would take the ships several days to get to the Myanmar area, the Navy was sending some of the group's helicopters and troops ahead over land.

"The Essex group ... either has or is (still) offloading some helicopters to be available in Thailand, because they could reach Myanmar in a very short — in a matter of hours from Thailand — with relief supplies," Gates said. "There are also I think six C-130s available."

The ships were to begin steaming toward Myanmar after the helicopters had left.

Officials said that although the military junta has not agreed to allow U.S. humanitarian assistance, it did ask for some other U.S. help — satellite pictures of the cyclone-devastated areas.

"They asked our defense attache at the embassy in Rangoon for some imagery and we provided it," said Marine Maj. Stuart Upton, a Pentagon spokesman.

Separately, the U.S. Senate passed a resolution urging humanitarian aid to Myanmar's people and asking Myanmar's government to remove restrictions on international aid groups.

Democratic Sen. John Kerry said in a statement that the cyclone "could be remembered as the moment when the United States and the world came to the aid of the Burmese people and made it clear that while we loathe the junta that has isolated Burma from the world and oppressed its citizens, we find common cause with the people of Burma and we will be there by their side at this difficult time."