Ammon News - WASHINGTON (Reuters) - Congressional committees are holding up a plan to send U.S. weapons to rebels fighting Syrian President Bashar al-Assad because of fears that such deliveries will not be decisive and the arms might end up in the hands of Islamist militants, five U.S. national security sources said.

Both the Senate and House of Representatives intelligence committees have expressed reservations behind closed doors at the effort by President Barack Obama's administration to support the insurgents by sending them military hardware.

None of the military aid that the United States announced weeks ago has arrived in Syria, according to an official from an Arab country and Syrian opposition sources.

Democrats and Republicans on the committees worry that weapons could reach factions like the Nusra Front, which is one of the most effective rebel groups but has also been labeled by the United States as a front for al Qaeda in Iraq.

Committee members also want to hear more about the administration's overall Syria policy, and about how it believes its arms plan will affect the situation on the ground, where Assad's forces have made recent gains.

Funding that the administration advised the congressional committees it wanted to use to pay for arms deliveries to Assad's opponents has been temporarily frozen, the sources said.

"As noted at the time we announced the expansion of our assistance to the Supreme Military Council, we will continue to consult closely with Congress on these matters," Bernadette Meehan, a spokeswoman for the White House National Security Council, said on Monday.

Technically, the administration does not need specific congressional approval either through public legislation or some kind of legislative sanction process to move ahead with the weapons plan. The president already has legal authority to order such shipments, several sources said.

However, under tacit rules observed by the executive branch and Congress on intelligence matters, administrations will not move ahead with programs like weapons deliveries to the Syrian opposition if one or both of the congressional intelligence committees express serious objections.

UNCONVINCED BY KERRY, CIA

Late last month, Secretary of State John Kerry and outgoing CIA Deputy Director Michael Morell briefed the intelligence committees in detail secretly about plans to arm the rebels in response to growing evidence that Assad's forces had used chemical weapons, the sources said.

After that briefing, members of both committees expressed dissatisfaction with the plan, the sources said.

Although initially the House committee voiced greater opposition than its Senate counterpart, after further consideration the Senate panel became concerned enough about the plan to write a letter to the administration raising questions about it, two of the sources said.

At the same time, the appropriations committees of both chambers, which also routinely review secret intelligence or military aid programs, raised doubts.

Syrian opposition sources and officials of governments in the region which support anti-Assad forces have begun to express puzzlement as to why new weapons shipments promised by Washington have not yet begun to arrive.

One Arab government official, speaking on condition of anonymity, expressed concern that the United States had only made the decision to provide weapons but had not yet determined exactly where to send them.

The White House announced in June that it would arm vetted groups of Syrian rebels, after two years of avoiding involvement in the civil war which has killed more than 100,000 people.

The only way the administration's plan will move forward, said the sources, is if congressional committees can work out a deal with the administration to resolve their concerns.

Anti-Assad groups have been calling for more advanced weaponry since the government launched a new offensive in central Syria with the help of the Lebanese militant group Hezbollah. Government forces are hammering the central city of Homs and have encircled rebel strongholds near the capital Damascus.

Over the weekend, the Syrian branch of the Muslim Brotherhood said it felt "abandoned and disappointed" that the United States and Europe had failed to deliver promised military support to the rebels.

A source in Washington who is close to the Syrian rebels also said he knew of no U.S. military aid that had been delivered to them.