The U.N. envoy to Libya urged the country's new leaders Thursday to act quickly to address major security challenges that have led to serious outbreaks of fighting and impose control over rival militias, tribes and armed backers of Moammar Gadhafi's ousted regime.

Tarek Mitri told the U.N. Security Council that a year after Gadhafi's overthrow, Libya has moved forward in its political transformation. He pointed to progress "towards stabilizing a country bereft of democracy, institutions, human rights and rule of law over the last 42 years" and Parliament's approval just over a week ago of a new Cabinet.

But Mitri said pressure is mounting on the new government from the Libyan people to move quickly to build and reform state security institutions including the military, police and judiciary.

He cited the widespread outrage and demonstration by some 30,000 residents of Benghazi following the Sept. 11 attack on the U.S. consulate that left U.S. ambassador Chris Stevens and three other Americans dead, and the public demand for the rule of law and the building of national security institutions.

"While this large outpouring of public support underscores the urgency of the issue, it provides the new government the opportunity to move swiftly and decisively in advancing security sector reform," Mitri said.

He told the council the government also faces the major task of deciding how to write a constitution and said the U.N. is urging that it "consult widely and move swiftly on the process of drafting" the document.

In his briefing, Mitri reported on recent fighting between the army and Gadhafi supporters in their last major stronghold of Bani Walid, between residents of the southern town of Shati and Tripoli-based armed brigades, and between rival brigades in Tripoli. He also said there have been serious security incidents in Benghazi, including car bombs and assassination attempts targeting security officials, a religious leader and a brigade commander.

"The events of Bani Walid, Tripoli, Benghazi underscore the need for rapid and effective policy decisions and practical measures to be taken in the area of security sector reform," he said.

Mitri said these should include a new "mechanism" to coordinate national security while the new army and police forces are built, the disarmament and reintegration of former revolutionaries, and a program to control weapons and ammunition, which have proliferated since last year's revolution.

The U.N. envoy also said he was concerned about the unresolved issue of people detained during the eight-month civil war.

"While the president and prime minister provided assurances that all detainees would be placed under the authority of the judiciary," Mitri said, "I stressed the need to institute measures to properly account for them and ensure their humane treatment and afford fair trial standards."

He said the U.N. continues to urge the ministries of justice, defense and interior to screen detainees and investigate cases of mistreatment and torture.