The United States is poised to send spy planes into Syria to track Islamic State jihadists whose advances have sparked international concern and American air strikes in neighbouring Iraq.
A US official confirmed the plans after Syria said on Monday it was willing to work with the international community, including Washington, to tackle extremist fighters.
But American officials said they did not plan to ask Damascus for permission for the flights, despite Syrian insistence that any military action on its soil must be coordinated in advance.
International concern about IS has been rising after a lightning offensive by the group through parts of Iraq and a string of brutal abuses, including the murder of US journalist James Foley.
The United Nations has accused IS and affiliated groups in Iraq of acts that could amount to crimes against humanity.
On Monday, Damascus said for the first time that it was willing to work with the international community, including the United States and Britain, to tackle IS and Al-Qaeda's Syrian affiliate Al-Nusra Front.
But Foreign Minister Walid Muallem also made it clear that Syria would not accept unilateral military strikes by the United States or any other country.
"Any violation of Syria's sovereignty would be an act of aggression," he said.
There would be "no justification" for strikes on Syrian territory "except in coordination with us to fight terrorism".
- Syria seeks cooperation -
Muallem said Syria was seeking cooperation within an international or regional coalition, or at the bilateral level within the framework of a recent UN Security Council resolution targeting IS and Al-Nusra.
On Tuesday, a security source in Damascus reiterated Syrian insistence on coordination after the US comments about possible surveillance.
"Any entrance into Syrian airspace without coordination would be considered an act of aggression," the source told AFP.
There have been few signs that the international community is willing to work publicly with President Bashar al-Assad's regime, which has been engaged in a brutal effort to put down an uprising that began in March 2011.
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Washington has accused Assad's regime of using chemical weapons against his own people and carrying out other widespread abuses.
The United States began air strikes against IS in neighbouring Iraq on August 8, in a bid to roll back its advances.
But US Chairman of the Joint Chiefs of Staff General Martin Dempsey has acknowledged that the group cannot be defeated "without addressing that part of the organisation that resides in Syria".
The White House says no decision has been taken on whether to carry out air strikes in Syria, although US aircraft have already entered Syrian airspace covertly at least once for a failed mission to rescue hostages including Foley.
- IS 'crimes against humanity' -
Foley's murder and advances by IS in both Syria and Iraq have heightened fears about the group, which emerged from Al-Qaeda's one-time Iraqi affiliate but has since parted ways.
UN human rights chief Navi Pillay said Monday that IS and affiliated groups in Iraq were "systematically targeting men, women and children based on their ethnic, religious or sectarian affiliation and are ruthlessly carrying out widespread ethnic and religious cleansing.
"Such persecution would amount to crimes against humanity," she said in a statement.
On Sunday, IS also cemented its control over an entire province in Syria for the first time, seizing the Tabqa military airport in a bloody battle that killed hundreds of people.
The air base was the last outpost controlled by the Syrian military in Raqa province, which has now become an IS stronghold.
The group has also advanced in recent days in northern Aleppo province and controls territory in the oil-rich eastern province of Deir Ezzor.
Syrian war planes launched at least 12 raids using precision rockets against IS positions in Deir Ezzor on Tuesday, according to the Britain-based Syrian Observatory for Human Rights NGO.
In Iraq, the group has seen its momentum curbed in some areas by Kurdish forces backed by American air strikes, but it still holds significant areas that federal troops are struggling to regain.
On Tuesday, a car bomb struck a crowded Baghdad intersection, killing 15 people and wounding at least 37, in an area where a suicide bomber targeted Shiite worshippers the day before, leaving 11 people dead.