UN leader Ban Ki-moon warned Tuesday that a western military strike could worsen the Syria crisis, but called on the divided UN Security Council to unite against the use of chemical arms.

UN leader Ban Ki-moon warned Tuesday that a western military strike could worsen the Syria crisis, but called on the divided UN Security Council to unite against the use of chemical arms.

Ban urged caution, without explicitly opposing any military operation.

"I take note of the argument for action to prevent future use of chemical weapons," Ban told a press briefing at UN headquarters before heading to the Group of 20 summit in Russia.

"At the same time, we must consider the impact of any measure on efforts to prevent further bloodshed," Ban said.

"Everything should be handled within the framework of the United Nations Charter," he added.

"The use of force is lawful only when in exercise of self-defense in accordance with Article 51 of the United Nations Charter and, or when the Security Council approves such action."

Syria wrote to Ban on Monday calling on him to prevent a US military strike, its UN envoy Bashar Jaafari said.

Among the five permanent members of the Security Council, Russia and China strongly oppose a military strike. The United States and France have backed a strike. The British government said it was ready to join action until its parliament voted against such a move.

Ban said the suspected use of chemical weapons in Syria meant the council now has a duty to overcome the bitter divisions.

The UN leader said that chemical weapons experts were working "urgently" to determine whether banned poison gas was used at Gouta, near Damascus, in an attack on August 21.

Ban said samples collected by UN examples in Syria last week would all be in laboratories in Europe by Wednesday.

The Syrian government has denied using chemical arms, blaming the reported attacks on opposition rebels.