US Secretary of State Hillary Clinton has said a working group will be set up with Turkey to coordinate the response to the deepening Syrian crisis.
She said after talks in Istanbul that she had discussed with Turkey's foreign minister how best to support opposition to President Bashar al-Assad's rule.
The US, she said, was also increasing aid for Syrian refugees.
Numbers of civilians fleeing violence in Syria have surged, according to the United Nations.
Turkey, like all of Syria's neighbours, is dealing with a growing humanitarian crisis as thousands of refugees flood across the border.
Rebels in the northern city of Aleppo say they are preparing a counter-attack after withdrawing from the strategic south-west district of Salah al-Din under heavy bombardment.
Turkey is currently supporting more than 50,000 Syrian refugees with more arriving every day.
In another development, Arab foreign ministers are to meet in the Saudi city of Jeddah on Sunday to discuss the Syrian crisis, Egyptian state TV reports.
Chemical weapons fear
Mrs Clinton told reporters that she and her Turkish counterpart, Ahmet Davutoglu, had discussed how to support the opposition in Syria in its "efforts to end the violence and begin the transition to a free, democratic Syria without Assad".
"Our number one goal is to hasten the end of the bloodshed and the Assad regime," she said.
A "range of contingencies" for Syria was discussed, including the possible use of chemical weapons by the Assad government, Mrs Clinton said.
She added that two million people were in need of humanitarian assistance in Syria. Mrs Clinton announced an extra $5.5m (£3.5m) in help for displaced Syrians in Turkey, saying this would bring US aid to Syria to $82m.
Among US concerns are reports that a growing number of al-Qaeda linked militants are fighting alongside rebels in Syria.
US intelligence officials quoted by AP news agency said at least 200 militants linked to al-Qaeda are already operating in Syria, and their numbers are growing as foreign fighters enter the country.
US officials fear they could establish a presence similar to that in Iraq, which could be hard to defeat if rebels eventually oust President Assad.
Analysts say it could be one reason why Washington has been reluctant to offer military assistance to the anti-Assad insurgency.
Sporadic violence was reported around Syria on Friday.
Journalists from Reuters news agency reported seeing residents fleeing Aleppo with cars packed with possessions, taking advantage of a lull in the fighting.
AFP news agency reported that a bakery in the city's eastern Tariq al-Bab district had been hit by a shell, killing about 12 people and injuring at least 20.
State news agency Sana also reported that government forces had repelled a rebel attack on Aleppo's international airport.
The opposition Syrian National Council said part of Aleppo's 13th-century citadel had been damaged by shelling.
Activists also reported fighting in suburbs of the capital, Damascus.
President Assad is facing down stiff international pressure to step aside despite months of anti-government protests and worsening violence.
He has suffered a string of high-status defections, including his former Prime Minister Riad Hijab, who fled to Jordan earlier this week.