* US collaborating with secret command center in Turkey

* Syrian rebels seen improving coherence, effectiveness

* Obama stops short of giving rebels lethal weapons

WASHINGTON - President Barack Obama has signed a secret order authorizing US support for rebels seeking to depose Syrian President Bashar al-Assad and his government, US sources familiar with the matter said.

Obama's order, approved earlier this year and known as an intelligence "finding," broadly permits the CIA and other US agencies to provide support that could help the rebels oust Assad.

This and other developments signal a shift toward growing, albeit still circumscribed, support for Assad's armed opponents - a shift that intensified following last month's failure of the U.N. Security Council to agree on tougher sanctions against the Damascus government.

The White House is for now apparently stopping short of giving the rebels lethal weapons, even as some US allies do just that.

But US 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.

'NERVE CENTER'

A US 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 (100 km) from the Syrian border, which is also home to Incirlik, a US air base where US 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 US 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.

On Tuesday, NBC News reported that the Free Syrian Army had obtained nearly two dozen surface-to-air missiles, weapons that could be used against Assad's helicopters and fixed-wing aircraft. Syrian government armed forces have employed such air power more extensively in recent days.

NBC said the shoulder-fired missiles, also known as MANPADs, had been delivered to the rebels via Turkey.

On Wednesday, however, Bassam al-Dada, a political adviser to the Free Syrian Army, denied the NBC report, telling the Arabic-language TV network Al-Arabiya that the group had "not obtained any such weapons at all." US government sources said they could not confirm the MANPADs deliveries, but could not rule them out either.

Current and former US and European officials previously said that weapons supplies, which were being organized and financed by Qatar and Saudi Arabia, were largely limited to guns and a limited number of anti-tank weapons, such as bazookas.

Indications are that US agencies have not been involved in providing weapons to Assad's opponents. In order to do so, Obama would have to approve a supplement, known as a "memorandum of notification, to his initial broad intelligence finding.

Further such memoranda would have to be signed by Obama to authorize other specific clandestine operations to support Syrian rebels.

Reuters first reported last week that the White House had crafted a directive authorizing greater US covert assistance to Syrian rebels. It was unclear at that time whether Obama had signed it.

OVERT SUPPORT

Separately from the president's secret order, the Obama administration has stated publicly that it is providing some backing for Assad's opponents.

The State Department said on Wednesday the US government had set aside a total of $25 million for "non-lethal" assistance to the Syrian opposition. A US official said that was mostly for communications equipment, including encrypted radios. .

The State Department also says the United States has set aside $64 million in humanitarian assistance for the Syrian people, including contributions to the World Food Program, the International Committee of the Red Cross and other aid agencies.

Also on Wednesday, the US Treasury confirmed it had granted authorization to the Syrian Support Group, Washington representative of one of the most active rebel factions, the Free Syrian Army, to conduct financial transactions on the rebel group's behalf. The authorization was first reported on Friday by Al-Monitor, a Middle East news and commentary website.

Last year, when rebels began organizing themselves to challenge the rule of Libyan leader Muammar Gaddafi, Obama also signed an initial "finding" broadly authorizing secret US backing for them. But the president moved cautiously in authorizing specific measures to support them.

Some US lawmakers, such as Republican Senators John McCain and Lindsey Graham, have criticized Obama for moving too slowly to assist the rebels and have suggested the US government become directly involved in arming Assad's opponents.

Other lawmakers have suggested caution, saying too little is known about the many rebel groups.

Recent news reports from the region have suggested that the influence and numbers of Islamist militants, some of them connected to al Qaeda or its affiliates, have been growing among Assad's opponents.

US and European officials say that, so far, intelligence agencies do not believe the militants' role in the anti-Assad opposition is dominant.

While US and allied government experts believe that the Syrian rebels have been making some progress against Assad's forces lately, most believe the conflict is nowhere near resolution, and could go on for years.

(Additional reporting by Tabassum Zakaria and Arshad Mohammed; Editing by Warren Strobel and Peter Cooney)