US President Barack Obama has authorised surveillance flights over Syria in order to gain intelligence on the activities of Islamic State (IS).
Correspondents say the move could mark the first step towards US air strikes inside Syria, where the jihadist group controls vast swathes of territory.
The US is already carrying out strikes against IS in neighbouring Iraq.
On Monday, the Syrian government said it would work with the international community in the fight against IS.
Western governments have so far rejected suggestions that they collaborate with President Bashar al-Assad in an attempt to counter the growing regional threat posed by IS.
They have repeatedly called on Mr Assad to step down since the beginning of the three-and-a-half year uprising against his rule, in which more than 191,000 people are believed to have been killed.
On Monday evening, US officials said Mr Obama had approved over the weekend reconnaissance flights by unmanned and manned aircraft.
One official later told the Associated Press that they had already begun.
The US military has been carrying out aerial surveillance of IS – an al-Qaeda breakaway formerly known as Isis – in Iraq for months and launched air strikes on 8 August.
The president cited the threat to US diplomats and military personnel and the humanitarian crisis in the north, where hundreds of thousands of people have fled their homes since June as IS fighters and allied Sunni rebels have taken control of dozens of cities, towns and villages.
Mr Obama has resisted taking military action in Syria, but Pentagon officials are said to have advised him that the only way the threat from IS can be fully eliminated is to go after the group there.
A spokesman for Gen Martin Dempsey, chairman of the US Joint Chiefs of Staff, said the Pentagon was “preparing options to address Isis both in Iraq and Syria with a variety of military tools including air strikes”.
The options reportedly include targeting IS leaders in and around their stronghold of the northern Syrian city of Raqqa, as well as in the east near the Iraqi border.
Last week, IS published a video showing it killing the American journalist James Foley, who was abducted in Syria in 2012. The group threatened to kill other US citizens it was holding in retaliation for US air strikes.
It later emerged that US special forces had attempted to rescue the hostages in July, but that they were not at the location in Syria where the military thought they were being held.
‘Ready for co-operation’
One Obama administration official told the New York Times that the US did not intend to collaborate with the Assad government or inform him in advance of any operation.
“It is not the case that the enemy of my enemy is my friend,” said Benjamin Rhodes, a deputy national security adviser. “Joining forces with Assad would essentially permanently alienate the Sunni population in both Syria and Iraq, who are necessary to dislodging [IS].”
On Monday, Syrian Foreign Minister Walid Muallem said his government was “ready for co-operation and co-ordination at the regional and international level to fight terrorism”.
But Mr Muallem warned the White House that it would view any unilateral military action as a breach of sovereignty and an “act of aggression”.
The Western-backed rebel Free Syrian Army, which is fighting IS across northern Syria, meanwhile said its commanders on the ground were ready to co-ordinate with the US.
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