The US has begun surveillance flights over rebel-controlled parts of Syria after presidential authorisation, officials said, a move that could pave the way for air raids against the Islamic State group.
A US official told the AP news agency early on Tuesday that the flights had started, while two other US officials said earlier that Barack Obama had approved the flights.
Martin Dempsey, the chairman of the joint chiefs of staff, said that the US wanted more clarity on the group in Syria, but declined to comment on the surveillance flights.
"Clearly the picture we have of ISIS on the Iraqi side is a more refined picture,'' said Dempsey, using one of the acronyms for the Islamic State group. "The existence and activities of ISIS on the Syrian side ... we certainly want to have more insights."
A White House source told Al Jazeera it would not comment on operational issues. However, it said the US was prepared to "use all the tools at our disposal... we don't restrict our options by geographic boundaries when it comes to the central mission of protecting our people".
Earlier on Monday, Syria's foreign minister, Walid Muallem, said that his country welcomed international support, but added that all efforts must be coordinated by the Syrian government.
Al Jazeera's Rula Amin, reporting from Beirut, said that "behind closed doors" Western intermediaries may have already started talking on how to fight the Islamic State group as a "common" target.
The US is already launching air raids on Islamic State targets in Iraq. The group announced last week that it had murdered an American journalist in Syria and was threatening more US hostages.
The Reuters news agency had earlier reported that Washington was planning to send surveillance aircraft, including unmanned drones, to gain a clearer picture of the Islamic State fighters, who have seized large swaths of territory in Syria and Iraq.
The Wall Street Journal said that the US central command, which oversees US forces in the Middle East, had asked for additional surveillance planes to be used in Syria.
Joshua Landis, an analyst on Syria, told Al Jazeera that advisers to Obama had been telling the president that the US would only be able to uproot the Islamic State group if they got involved in Syria.
But the dilemma Obama was facing was the lack of partners in Syria, Landis said. Neither the Syrian regime nor the fragmented moderate rebels could be seen as reliable allies.
The UN Security Council passed a rare unanimous resolution on August 15 intended to weaken armed groups in Iraq and Syria by choking off their funding and stemming the flow of foreign fighters.
The resolution targeted both the Islamic State group and the Nusra Front, which is al-Qaeda's Syrian affiliate.
Syria's government considers these two groups, and all others fighting to overthrow president Bashar al-Assad, to be terrorists.