President Barack Obama has signed a secret order authorizing U.S. support for the Syrian rebels, U.S. sources familiar with the matter tell Reuters.

Obama's order, approved earlier this year and known as an intelligence "finding," broadly permits the CIA and other U.S. agencies to provide support that could help the rebels oust President Bashar al-Assad, Reuters reports.

This and other developments signal a shift toward growing, albeit still limited, support for Assad's armed opponents - a shift hastened by the failure of the U.N. Security Council to agree on tougher sanctions against the Damascus government, thanks to vetoes by Russia and China.

The White House is for now apparently stopping short of giving the rebels lethal weapons, but U.S. allies are doing so.

But U.S. and European officials have said that there have been noticeable improvements in the coherence and effectiveness of Syrian rebel groups in the past few weeks. That represents a significant change in assessments of the rebels by Western officials, who previously characterized Assad's opponents as a disorganized, almost chaotic, rabble.

Precisely when Obama signed the secret intelligence authorization, an action not previously reported, could not be determined.

The full extent of clandestine support that agencies like the CIA might be providing also is unclear.

White House spokesman Tommy Vietor declined comment.

A U.S. government source acknowledged that under provisions of the presidential finding, the United States was collaborating with a secret command center operated by Turkey and its allies.

Last week, Reuters reported that, along with Saudi Arabia and Qatar, Turkey had established a secret base near the Syrian border to help direct vital military and communications support to Assad's opponents.

This "nerve center" is in Adana, a city in southern Turkey about 60 miles from the Syrian border, which is also home to Incirlik, a U.S. air base where U.S. military and intelligence agencies maintain a substantial presence.

Turkey's moderate Islamist government has been demanding Assad's departure with growing vehemence. Turkish authorities are said by current and former U.S. government officials to be increasingly involved in providing Syrian rebels with training and possibly equipment.

European government sources said wealthy families in Saudi Arabia and Qatar were providing significant financing to the rebels. Senior officials of the Saudi and Qatari governments have publicly called for Assad's departure.