Syria has relocated some of its chemical weapons in the interest of security, but its main storage sites remain secure and under government control according to U.S. Defence Secretary Leon Panetta, citing intelligence reports.

"There has been some intelligence that with regards to some of these sites that there has been some movement in order for the Syrians to better secure ... the chemicals," Panetta said Friday morning during a press conference at the Pentagon. "So while there's been some limited movement, again the major sites still remain in place, still remain secure."

Syria is believed to have stockpiles of nerve and mustard gas. The regime of President Bashar al-Assad admitted earlier this year it has unspecified chemical weapons, though it later back-tracked on those remarks.

The regime's war with the Syrian rebels has stoked fears that the weapons could fall into other hands.

Panetta's remarks come as Western and allied nations meet in New York, looking to unite the fractured rebel forces.

U.S. Secretary of State Hillary Clinton is hosting talks among the Friends of Syria — a coalition which includes the United States, the European Union and the Arab League — on the sidelines of the UN General Assembly, seeking to encourage better co-operation among the rebel groups.

The UN Security Council remains deadlocked on on efforts to halt the 18-month long civil war, which activists say has led to more than 30,000 deaths.

The talks do not include Syrian allies Russia, China or Iran. Russia and China have vetoed three Western-backed resolutions aimed at pressuring Assad to enter negotiations on a political transition, paralyzing the UN's most powerful body and denting chances of any progress during the General Assembly.

French Foreign Minister Laurent Fabius has said it is "shocking" that the Security Council had been unable to act, while British Prime Minister David Cameron denounced the deaths of Syrian children as "a stain on those who have failed to stand up to these atrocities," a reference to Russia and China.

Clinton has decried al-Assad's "murdering of his own people," while Russia's Foreign Minister Sergey Lavrov — who will address the General Assembly on Friday — has accused the U.S. and other countries of encouraging terrorism in their stance on Syria.

Meanwhile, the UN's top human rights body is extending the mission of its independent expert panel probing alleged war crimes in the conflict, while rebel and government forces saw renewed fighting in Aleppo.

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U.S. Secretary of State Hillary Clinton will on Friday host a meeting of Western and allied nations, urging more co-operation among the Syrian rebels. (John Minchillo/Associated Press)

Fighters from the main rebel group known as the Free Syrian Army launched a new offensive on Friday, looking to drive regime forces from their strongholds and end the prolonged stalemate over Syria's largest city and commercial capital.

The rebels are calling the latest push the "decisive battle" according to activists.

"The city is witnessing one of the most violent days. All fronts are on fire," said Aleppo-based activist Baraa al-Halabi.

The Syrian military sent text a message on cellular telephones to members of the armed rebellion reading: "Game over."

The reports could not be independently confirmed because the government has imposed tight restrictions on the media.

The city of 3 million, once a bastion of support for al-Assad, has emerged as a key battleground in Syria's civil war. Its fall would give the opposition a major strategic victory with a stronghold in the north near the Turkish border. A rebel defeat, at the very least, would buy the regime more time.

Back at the UN, the human rights panel led by Brazilian professor Paulo Sergio Pinheiro has blamed Syria's government forces for the majority of serious abuses since the uprising began in March 2011.

Last week it submitted a confidential second list of suspected war crimes perpetrators to the UN human rights office. The panel's mandate was due to expire at the end of the month.