Marine Corps aviators are not as ready as they need to be, the head of the Corps told senators on Thursday.

Commandant Gen. Robert Neller said the readiness of the service's planes is "below an acceptable level," though specifics vary among types of aircraft.

"In the aggregate, it's improving, but it's not where we need to to be," Neller said at a Senate Armed Services Committee hearing.

Neller said more funding would get the service back up to an acceptable level more quickly.

All four service chiefs said they will not be able to defend the country if sequestration budget caps are left in place. All the service leaders told Sen. Lindsey Graham, R-S.C., that they rank sequestration as a threat to the military alongside other threats such as nation-states or terrorists.

"Would you agree with me, general, that the Congress is going to shoot down more planes than any enemy that we can think of in the near term?" Graham asked.

"Potentially," Air Force Chief of Staff Gen. David Goldfein responded.

Graham pushed further, asking leaders if they would rank Congress and the president as threats to the nation's security if officials fail to undo sequestration.

Sen. John McCain, R-Ariz. and chairman of the Senate Armed Services Committee, gave the generals an out, saying they didn't have to respond to that question.