WASHINGTON - The next days and weeks in Libya are "critical" as the "future is not guaranteed," US Secretary of State Hillary Clinton said Thursday, pledging full American support.

"There can be no place in the new Libya for revenge attacks and reprisals," Clinton said in a written statement. "Libya's future will be peaceful only if the leaders and people of Libya reach out to each other in a spirit of peace."

Clinton urged the "new Libya" to be firm against "violent extremism" on Thursday, saying the triumphant rebels had obligations to the international community.

"We will look to them to ensure that Libya fulfills its treaty responsibilities, that it ensures that its weapons stockpiles do not threaten its neighbors or fall into the wrong hands, and that it takes a firm stand against violent extremism," Clinton said in a written statement.

Libyan rebels have transferred their political leadership to the capital Tripoli from their base in Benghazi, a senior official of the National Transitional Council announced Friday.

"I declare the beginning and assumption of the executive committee's work in Tripoli," committee vice-chairman Ali Tarhuni told a press conference. "Long live democratic and constitutional Libya and glory to our martyrs."

Tarhuni said he made his declaration "in the name of God, our revolutionaries and martyrs" and in line with the directions of the NTC and the executive committee.

He said NTC head Mustafa Abdel Jalil would arrive in Tripoli as soon as the security situation permitted.

Tarhuni, minister of oil and economics in the provisional administration, also named the holders of key cabinet posts including interior minister, information minister and infrastructure minister, as well as the head of security for Tripoli.

He called on forces loyal to Moamer Kadhafi to lay down their arms, and the police to return to work, promising them safety and lawful treatment.

"Put your weapons down and go home. We will not take revenge. Between us and between you is the law. I promise you will be safe."

Yellow cake safe

The United States said Thursday that Libya's stockpiles of mustard agent and uranium yellowcake -- potential ingredients for chemical and nuclear weapons -- are secure.

State Department spokeswoman Victoria Nuland said US envoy Chris Stevens was speaking with Libyan rebels about their plans to assume control of a site in Tajjura where yellowcake is kept and a mustard agent storage site in Waddan.

"All sensitive elements of Libya's nuclear program, including everything that Libya received from the (Pakistan-based) AQ Khan network, were removed in early 2004," Nuland told reporters.

"The last of the highly-enriched uranium, the bomb-making fuel, was removed from Libya in 2009," she said.

She said Libya's supply of yellowcake is "safeguarded" at the Tajjura nuclear research facility.

Yellowcake is made from raw mined uranium to be transformed into uranium tetrafluoride (UF4) and then into uranium hexafluoride (UF6), a feed gas for the actual process of enrichment. Enriched uranium can be used to fuel nuclear reactors or make atomic bombs.

"We are able, through our national technical means, to assert that we believe that it is secure and .... in any case, Libya doesn't have the means right now to turn yellowcake into anything dangerous," she added.

National technical means refers to the use of spy satellites and other intelligence-gathering methods.

The now fugitive Libyan strongman Moamer Kadhafi's stockpile of mustard agent meanwhile is "now stored at the Waddan Ammunition Reservation," Nuland said.

"It is inside massive steel containers within heavy bunkers. These bunkers were sealed by the Organization for the Prohibition of Chemical Weapons, the OPCW," Nuland added.

"Our judgment is that they remain secure," she said, adding the chemicals are not ready to be mounted on weapons.

"They can't be converted on a dime. And they're in these massive drums inside a heavy bunker. And we are able to monitor the security with national technical means," she added.

The State Department and Pentagon said Wednesday that Libya's stockpiles of missiles and chemical weapons were "secure" but that an arsenal of thousands of portable shoulder-launched missiles remained a cause for concern.