The United States said Wednesday that Libya's stockpile of missiles and chemical weapons are "secure" but that an arsenal of thousands of shoulder-launched missiles remained cause for concern.

State Department spokeswoman Victoria Nuland said US officials have been "monitoring known missile and chemical agent storage facilities since the start of this conflict" in February.

"We believe that these known missile and chemical agent storage facilities remain secure, and we've not seen any activity, based on our national technical means, to give us concern that they have been compromised," she told reporters.

"But that monitoring will continue," Nuland said.

Pentagon spokesman Colonel Dave Lapan meanwhile replied "yes" when asked if sites containing chemical weapons, including over 10 tons of mustard gas, were safe.

But he declined to offer more details, only saying that "clearly those are dangerous agents and weapons… we continue to monitor that."

As rebel forces declared they had defeated Moamer Kadhafi's regime and were now trying to secure the whole of Tripoli, Lapan told reporters there were no plans to send US troops in to secure the chemical weapons' sites.

Although Kadhafi's regime retained the mustard gas, it lacked the military means to launch an attack with the chemical, according to arms control experts.

Kadhafi joined the Organisation for the Prohibition of Chemical Weapons in 2004 after renouncing weapons of mass destruction in December 2003, but still had to eliminate 11.25 tons of mustard gas when the uprising to remove him from power began in February.

All 3,563 munitions — such as bombs, shells and missiles — that could serve as a delivery vehicle for mustard gas have been destroyed, according to the OPCW.

Lapan said the United States was also concerned about a plethora of conventional arms and ammunition, including shoulder-launched surface-to-air missiles.

The shoulder-launched missiles in particular posed a potential danger, he said.

"They remain a concern, because of their portability."

General Carter Ham, head of US Africa Command, told lawmakers in April that there were an estimated 20,000 shoulder-launched missiles in Libya.

Members of Congress have voiced worries that some of the portable missiles could fall into the hands of Islamist extremists with ties to the Al-Qaeda network.

Amid speculation about Kadhafi's whereabouts, Lapan reiterated that the Pentagon believed he was still in the country.

Nuland said the US has given $3 million to two non-government organizations to help the rebel National Transitional Council (NTC) secure conventional weapons stockpiles as well as dispose of landmines and unexploded munitions.

She said the organizations, MAG International and the Swiss Foundation for Demining, have been working with the NTC in areas under the council's control to secure such weapons, including the shoulder-launched missiles.

Nuland said she was unable to offer details on the scope of the problem of missiles known as MANPADs (Man-Portable Air Defense Systems).

But she said two teams of US government experts have visited countries in the region to make sure the shoulder-launched missiles are not being smuggled outside of Libya.

"I think the concern is not only about remaining MANPADS in Libya but also about their proliferation, which is why we're working with all of the neighbors," Nuland said.

"This is something that we have been working on and that we will continue to work on as the (NTC) takes control of Libya. And they want to continue to work on it with us and with the international community," she said.