Defense chiefs on Thursday defended the military response to a militant attack on a US mission in Libya, but warned looming deep budget cuts threaten America's ability to protect itself.

"I firmly believe that the Department of Defense and the US armed forces did all we could do in the response to the attacks in Benghazi," Defense Secretary Leon Panetta told the Senate Armed Services Committee.

But, in a hearing lasting over four hours, he warned that the US military is not an on-call service ready to rush to every emergency around the world.

He also urged lawmakers to help remove the threat of deep automatic budget cuts set to hit the Defense Department from March 1, calling them one of the greatest risks to America's national security.

Panetta and the chairman of the Joint Chiefs of Staff, General Martin Dempsey, were grilled by senators probing what happened during the deadly September 11 militant attack on the US mission in Benghazi.

Despite there being US bases in the Africa region and in Italy, Panetta said there was not enough time to scramble resources to Benghazi as the mission and later a nearby annex came under attack.

An unmanned surveillance drone did arrive on the scene an hour and 11 minutes after the start of the assault, but it would have taken a fixed-wing aircraft between nine to 12 hours to get there.

"The United States military, as I've said, is not and frankly should not be a 911 service capable of arriving on the scene within minutes to every possible contingency around the world," Panetta said.

"The US military has neither the resources nor the responsibility to have a firehouse next to every US facility in the world."

Panetta also stressed there had been "no specific intelligence" of an attack on the mission in Benghazi, in which the US ambassador to Libya, Chris Stevens, and three other Americans were killed.

"Frankly, without an adequate warning, there was not enough time, given the speed of the attack, for armed military assets to respond," Panetta said.

But veteran Senator John McCain took issue with Dempsey's written statement in which he said that "our military was appropriately responsive."

"All of this is a result of the so-called light footprint," McCain argued, saying the US military should have considered earlier general warnings about deteriorating security in Libya and placed aircraft and troops on standby in a nearby Mediterranean base on Crete.

"It was almost predictable... that bad things were going to happen in Libya because here was a fledgling government that had never governed before, without the assistance that we could have provided them."

Dempsey retorted that on the day, the 11th anniversary of the September 11, 2001 attacks, he was also busy with "pretty significant intel threat streams" against Sanaa, Khartoum, Islamabad, Peshawar, Kabul and Baghdad.

However, Panetta assured lawmakers the Pentagon, working with the State Department, was putting in place new measures in the wake of the attack, including plans to base 1,000 more Marines at US missions around the world.

Dempsey agreed that training programs based out of the Africa Command Center (AFRICOM) should be expanded, highlighting the threat network in North and West Africa posed by groups, some of which are inspired by Al-Qaeda.

They have moved into "ungoverned space... where we have to be very careful not to allow these movements to take sanctuary."

But Panetta warned of the potential risk if lawmakers fail to reach a deal with President Barack Obama to avert automatic budget cuts on March 1, which would slash the defense budget by $46 billion.

The threat of what is called sequestration is "one of the greatest security risks we are now facing as a nation," Panetta said.

"This budget uncertainty could prompt the most significant military readiness crisis in more than a decade," he warned.