UN Secretary-General Ban Ki-moon

Arab League foreign ministers have postponed a meeting in Saudi Arabia to debate the crisis in Syria.

The ministers had been due to discuss a new envoy to Syria to replace Kofi Annan, who resigned earlier this month, reports the BBC.

The US Secretary of State Hillary Clinton has said the US and Turkey are working together on detailed plans to support the Syrian opposition.

Fighting has continued in the Syrian capital, Damascus, and the second city of Aleppo.

Speaking on a visit to Istanbul, Mrs Clinton said both the US and Turkey were making preparations to respond to the possible collapse of President Bashar al-Assad's government, the use of chemical weapons and increases in the number of cross-border refugees.

Annan resigned from his position as UN-Arab League envoy to Syria earlier this month, after his proposed six-point peace plan failed to come into effect and violence escalated.

His replacement had been due to top the agenda for foreign ministers who had been due to gather on Sunday in the Saudi city of Jeddah.

But the Arab League's deputy secretary general, Ahmed Ben Helli, told reporters the meeting has been postponed, and no new date had yet been set.

He did not say why the meeting had been delayed.

The man tipped by diplomats to be given the job is veteran Algerian diplomat Lakhdar Brahimi.

The mandate of the United Nations observer mission in Syria - which now comprises some 150 observers - is due to run out in a week's time.

But UN Secretary-General Ban Ki-moon says there still need to be people on the ground to make impartial assessments of the military situation.

The Security Council will discuss the issue on Thursday, but there is little consensus on the council, with Syrian ally Russia calling for an extension and the US sceptical about prolonging the mission.

Instead, the US is taking steps outside the structures of the UN to support Syrian opposition groups, such as the setting up of the working group with Turkey announced by Mrs Clinton in her meeting with her Turkish counterpart Ahmet Davutoglu on Saturday.

"Our number-one goal is to hasten the end of the bloodshed and the Assad regime," she said.

"Our intelligence services, our military have very important responsibilities and roles to play so we are going to be setting up a working group to do exactly that."

A "range of contingencies" was discussed, including the possible use of chemical weapons by the Assad government, Mrs Clinton added.