The US Defence Secretary, Leon Panetta, has said Libya faces a long and difficult road in moving on from 42 years of Colonel Gaddafi's rule.
Mr Panetta was speaking while on a visit to Tripoli.
His visit coincides with the decision by the United Nations Security Council to lift sanctions on the Libyan central bank, clearing the way for its overseas assets to be unfrozen.
The move will give the new government access to tens of billions of dollars.
Addressing a joint news conference with Prime Minister Abdurrahim El-Keib, Mr Panetta said Libya faced tough challenges in uniting the forces that toppled Gaddafi, securing arms caches and building an army, police force and democratic institutions.
"This will be a long and difficult transition, but I am confident that you will succeed," he said.
Asked about the militias that wield the real power on Libya's streets, Mr Panetta said: "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.
"I have a good sense that they know how to deal with it."
The UNSC has lifted sanctions on Libya's central bank and a subsidiary, clearing the way for their overseas assets to be unfrozen to ease a cash crisis.
The Central Bank of Libya and the Libyan Foreign, an offshore institution wholly owned by the central bank, have been taken off the council's sanctions list.
The council froze Libyan assets abroad estimated at $150bn when an uprising against the rule of leader Muammar Gaddafi erupted in February.
Most of that sum has remained beyond the reach of the oil-rich country's new rulers.
Gaddafi's 42-year rule collapsed when his forces fled Tripoli in August, and the last of the fighting in Libya ended in October when he was captured and killed by rebels.
Yet by late November only about $18bn in seized assets had been released by special provisions of the UNSC's Libya Sanctions Committee, and diplomats said only about $3bn of that had been made available to Tripoli.
A UN resolution in September eased sanctions on Libya, removing them from the national oil company but leaving them largely in place on the central bank and LFB because of legal problems over unfreezing their foreign assets.
Last week, senior figures in Libya's new leadership wrote to the committee asking it to delist the two banks, which had been sanctioned along with two Libyan investment authorities.
The appeal stated that the move was "essential for the economic stability of Libya; for confidence in the banking sector; for the smooth execution and settlement of both domestic and international banking transactions; and to underpin the social and microeconomic stability of the new Libya."
Frustration at the delay in releasing the assets has been growing inside Libya, where the interim government says it urgently needs the cash to pay the wages of public sector workers and to start re-building state institutions.
The freezing of Libyan assets was part of a package of sanctions by the 15-nation council intended to put pressure on Gaddafi's government to stop attacking civilian protesters.
