LONDON (AFP) - The United Nations rights chief said on Saturday that the international community was hesitating to take action on Syria because countries were weighing up whether any military intervention would be worth it.

Urging that some sort of international action be taken against Syrian President Bashar al-Assad, Ms Navi Pillay repeated her call for him to be investigated for "crimes against humanity and war crimes".

Asked by Britain's Channel 4 television whether it would be difficult for the United Nations to intervene in a place like Syria, she said: "It's an intergovernmental decision on what kind of action: intervention, peacekeeping, military intervention or a referral to the international criminal courts.

"We urge that action be taken immediately. If there is doubt or hesitation it is because people are assessing the value of military intervention in places like Libya, Syria and Afghanistan. It could become a long, drawn out war with no guarantees that civilians would not be harmed in that process."

The UN high commissioner for human rights said Mr Assad should face international justice. "War crimes are being committed by President Assad's forces - his security forces - and other groups allied to him," the South African said.

"He is responsible and there should be a referral by the Security Council to the International Criminal Court." The former ICC judge said Mr Assad had demonstrated a "manifest failure" of his obligation as a head of state "to protect your citizens and that would be the opening for international intervention - of whatever kind."

Rebels pressed an offensive in northern Syria on Saturday, attacking Aleppo airport and two airbases, as a rights watchdog and residents reported hundreds of people held in a string of sectarian kidnappings.

French Foreign Minister Jean-Yves Le Drian, speaking at a security forum in Abu Dhabi, called for urgent action to bring about a power transfer that excludes Mr Assad.