The White House has said that President Barack Obama has so far made no decision on whether to launch air strikes on Islamic State militants in Syria.
It comes amid rising expectations of US action following American attacks against the jihadist group in Iraq.
Earlier, Syria said it would co-operate in any international efforts to fight Islamic State militants in the country, after the US suggested it may extend the battle against the group into Syrian territory.
Foreign Minister Walid al-Moualem, whose government has been shunned by the West, presented his country as a vital partner in the war against the Islamic State.
IS has seized wide areas of Syria and Iraq.
"Syria, geographically and operationally, is the centre of the international coalition to fight Islamic State," Mr Moualem said in a televised news conference.
"States must come to it if they are serious in combating terrorism," he added.
Asked about the prospect of US air strikes against Islamic State inside Syria, Mr Moualem said his government was ready to cooperate with any country fighting militants.
But he said any air raids mounted without approval would be viewed as hostile acts.
"Anything outside this [co-operation] is considered aggression," he said.
The White House signalled on Friday it was considering taking the fight against Islamic State into Syria after days of air strikes against the group in Iraq and the beheading of an American journalist.
But the US has also supported a more than three-year-old insurgency against Syrian President Bashar al-Assad and there has been no sign of any shift in US policy towards the Syrian leader.
"He's part of the problem," Ben Rhodes, President Barack Obama's deputy national security adviser, said in a broadcast interview last week.
Last year the US came close to bombing Syria after accusing Mr Assad's forces of using chemical weapons.
Islamic State, an offshoot of al-Qaeda, has emerged as the strongest group in the insurgency against President Assad.
It controls roughly a third of northern and eastern Syria and has since grabbed territory in neighbouring Iraq, declaring a cross-border "caliphate".