Hillary Clinton, the US secretary of state, has said her country is setting up a working group with Turkey to plan for worst-case scenarios in Syria, including a possible chemical weapons attack on the government's opponents.
In a joint news conference in Istanbul with Ahmet Davutoglu, Turkey's foreign minister, Clinton said that the group will co-ordinate military, intelligence and political responses to any potential fallout in Syria.
The US and Turkey agreed on the need to plan for "the horrible event that chemical weapons [are] used," Clinton said on Saturday.
"What would that mean in terms of response, humanitarian and medical emergency assistance and, of course, what needs to be done to secure those stocks from ever being used or falling into the wrong hands?"
In July, Syrian foreign ministry spokesman Jihad Makdissi ruled out the use of chemical weapons on Syrians, saying they would only be used in the event of foreign military intervention in Syria.
Clinton announced an additional $5m in aid for those fleeing the unrest in Syria, which monitoring groups say has now claimed more than 21,000 lives.
The announcement brings the total amount of US aid to more than $80m, Al Jazeera's Andrew Simmons reported from Istanbul.
"No matter how much money you throw at this emergency, getting it to the people is a different matter. The question was put to Clinton [of whether] she is considering a no-fly zone. She didn't answer directly, but she did confirm that it was on the table ... but no decisions have been taken yet," he said.
"Now it could be the only way to get the aid to the people and to assist the opposition."
Arab foreign ministers, meanwhile, are due to hold an emergency meeting in Saudi Arabia on Sunday to discuss the Syrian crisis.
Clashes in Damascus
The diplomatic developments came as forces loyal to President Bashar al-Assad clashed on Saturday with rebels in the heart of the capital Damascus, near the central bank, residents and state television said.
An explosion was followed by fighting which appeared to be spreading, said one resident, who asked not to be named for fear of arrest.
In-depth coverage of escalating violence across Syria
"The explosion was huge. There has been fighting for the past half an hour along Pakistan Street. I am very close. Can you hear that?" she asked, to the sound of a loud bang.
State television said that "terrorists" had detonated a bomb in Merjeh, an area near the central bank.
It said fighters were "shooting at random to spark panic among citizens and the authorities hunted the terrorist groups".
Fighting had also been reported by anti-government groups in other districts, including Tadamun, Qaboon and Qadam districts.
Syrian troops say they have purged rebels from the capital after intense, week-long battles last month. But rebels continue to stage hit-and-run attacks and are active in the suburbs around the city.
In Aleppo, Syria's second city, the battle for control of the city continued on Saturday, with government forces shelling several districts, according to the UK-based Syrian Observatory for Human Rights, an anti-Assad rights group.
Among those districts shelled were Saif al-Dawla, Bustan al-Qasr, al-Masharaqa and Malak. They also continued their offensive on the Salaheddin district, which has been the site of some of the fiercest fighting since rebels took over control of it late last month.
Local rebel commanders said on Saturday that despite taking heavy fire, they would continue fighting.
"Fierce fighting has continued without respite for the past 24 hours as the army tries to push us out of the neighbourhood," Abdel Qader Saleh told AFP by telephone.
Saturday's fighting came after state media reported a failed rebel attack on Aleppo's international airport and shelling in several other districts on Friday.