US Defence Secretary Leon Panetta told Libya's leaders on Saturday they faced a long, hard road in moving on from 42 years of one-man rule and uniting rival militias that still hold the streets in the oil-producing North African state.

Panetta, the first US defence chief ever to visit Libya, said Washington stood ready to help but offered no specific aid to a leadership struggling to stamp its authority two months after the capture and killing of Muammar Gaddafi.

He warned of tough challenges ahead in uniting the armed groups that emerged from the war, in securing arms caches and building an army, police and democratic institutions.

"This will be a long and difficult transition, but I am confident that you will succeed," the defence secretary said at a news conference after meeting interim Prime Minister Abdurrahim El-Keib and Defence Minister Osama Al-Juwali.

The authority of Libya's interim government is being challenged by militias who took Tripoli in August, six months after the start of a rebellion against Gaddafi that drew Nato into an air war.

Some withdrew after Gaddafi was killed in October, but others remain, heavily armed and holding out for a share of the power they say they are owed.

"I'm confident they (the interim leaders) are taking the right steps to reach out to all of these groups and bring them together so they will be part of one Libya and one defence system," Panetta said.

Clashes between militias and rival tribes since Gaddafi's ouster are threatening to spiral out of control in the absence of a fully-functioning government or national security force to unite the thinly populated desert country.

Prime Minister Keib promised job programs and other "opportunities" to help coax the militias off the streets.-Reuters