Foreign ministers from the Arab League are due to meet in the Saudi city of Jeddah to debate the crisis in Syria.
The ministers are expected to discuss a new envoy to Syria to replace Kofi Annan, who resigned earlier this month.
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.
Mr 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.
On the agenda for foreign ministers attending Sunday's emergency meeting in Jeddah will be Mr Annan's replacement - tipped by diplomats to be the veteran Algerian diplomat Lakhdar Brahimi.
At the table will be envoys from Saudi Arabia and Qatar - leading backers of the rebels in Syria.
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.
US Secretary of State Hillary Clinton: "We are continuing to increase pressure from the outside"
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.
Meanwhile, inside Syria fighting is continuing.
Syrian state TV said authorities were hunting "terrorists" who had set off a bomb in Marjeh, an exclusive district of Damascus near the central bank, and who were "shooting at random to spark panic among citizens".
At about the same time, another blast went off near Tishrin Stadium close by, reported state news agency Sana.
Hours later, Sana reported that a bus had been attacked in a Damascus suburb, said AP news agency, with six passengers from the central province of Hama killed. It blamed the attack on the "terrorists".
Violence erupted again between the rebel Free Syrian Army (FSA) and government forces in the country's largest city, Aleppo.
Activists said the army pounded areas south-west of Salah al-Din, from which the rebels retreated on Thursday.
Reports from Syria are difficult to confirm because of restrictions on reporters working there.
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