A senior aide to President Barack Obama did not rule out on Wednesday the eventual creation of a no-fly zone over a patch of Syria that increasingly appears to be controlled by anti-government rebels.

Some Republican critics of Obama’s handling of the Syria crisis have been advocating international enforcement of a no-fly zone to prevent Syrian warplanes from operating over designated zones, as well as more directly arming the opposition forces fighting to overthrow President Bashar al-Assad.

The United States has so far declined to provide weapons directly to a fragmented opposition as U.S. officials say it is difficult to identify factions and who they represent. Instead, Washington has focused on humanitarian aid, communications gear and other non lethal support.

“The United States government always looks at situations and looks at what types of scenarios might unfold, and then accordingly looks at what types of contingency plans might be available to deal with certain circumstances,” said John Brennan, Obama’s senior counter-terrorism adviser.

“So rest assured that various options that are being talked about in the press, and sometimes being advocated, these are things that the United States government has been looking at very carefully, trying to understand the implications, trying to understand the advantages and the disadvantages,” he said.

Asked during an appearance at the Council on Foreign Relations more pointedly about a no-fly zone, Brennan replied: “I don’t recall the president ever saying that anything is off the table.”

Brennan’s remarks came as the Obama administration is incrementally increasing its backing for Syrian rebels, and accelerating planning for a post-Assad Syria.

Secretary of State Hillary Clinton is due to have talks on Syria in Turkey on Saturday. She said as recently as Tuesday that the rebels reportedly held territory from northern Aleppo, Syria’s largest city, to the Turkish border.