By Mark Hosenball and Patricia Zengerle
WASHINGTON (Reuters) - The U.S. and its allies have "no smoking gun" proving Syrian President Bashar Al-Assad personally ordered his forces to use chemical weapons to attack a rebel-held Damascus neighborhood, U.S. national security officials said on Thursday.
In secret intelligence assessments and a still-unreleased report summarizing U.S. intelligence on the alleged gas attack on August 21, U.S. agencies express high confidence that Syrian government forces carried out the attack, and that Assad's government therefore bears responsibility, the officials said.
"This was not a rogue operation," one U.S. official said.
However the evidence does not prove that Assad himself ordered that chemical munitions be used, according to the officials, who spoke on condition of anonymity.
Evidence that forces loyal to Assad were responsible goes beyond the circumstantial to include electronic intercepts and some tentative scientific samples from the neighborhood which was attacked, officials said.
U.S. Secretary of State John Kerry, Secretary of Defense Chuck Hagel and Admiral James A. Winnefeld Jr, Vice Chairman of the Joint Chiefs of Staff, are among senior U.S. officials who will brief senior members of Congress on Thursday about the situation in Syria and related intelligence assessments, congressional aides said.
President Barack Obama's national security adviser, Susan Rice, and Director of National Intelligence James Clapper will also participate in the briefing. With many members of Congress out of Washington during the summer recess, the briefing will be held in a telephone conference call at 6 p.m. EDT (2200 GMT), according to the aides.
They said the briefing had originally been meant to be classified, but too many members of Congress were unable to get to secure telephone lines for it to include top secret material.
While Obama has not yet announced a decision on military action, he has left little doubt the choice was not whether but when to punish Assad's government for the attack, in which hundreds of people died.
Some Republican lawmakers and even some of Obama's fellow Democrats have complained that they have not been properly informed.
USE OF CHEMICAL WEAPONS
Thursday's briefing is for leaders of the Senate and House of Representatives as well as the chairmen and ranking members of national security committees, including foreign relations, intelligence and armed services.
During the week, U.S. government spokespeople have made increasingly emphatic statements declaring that chemical weapons were used and that Assad's government, rather than rebel forces, were responsible for using them.
"This was a massive, large-scale ... multiple-faceted attack against a wide swath of area using very sophisticated rockets, very sophisticated delivery systems that were armed with chemical weapons. There is one party in Syria who has the capability to do that, and it's the Assad regime," State Department spokeswoman Marie Harf said on Wednesday.
Harf added that U.S. officials "ultimately, of course, hold President Assad responsible for the use of chemical weapons by his regime against his own people, regardless of where the command and control lies."
U.S. security sources and sources close to allied governments say evidence suggests that the initial decision to use chemical weapons may have been made by a field commander rather than in an order from the highest level of the Syrian government.
In a paper published on Thursday, the British government's Joint Intelligence Committee said: "Permission to authorize CW (chemical weapons) has probably been delegated by President Assad to senior regime commanders, such as, but any deliberate change in the scale and nature of use would require his authorization."
The name of the commander or commanders was redacted from the public version of the paper.
According to a former U.S. official who is an expert in the region, one possibility is that the Syrian ground commander in charge of clearing out the area which was attacked, under heavy pressure from superiors, may have made the initial decision to use chemical weapons before sending in ground troops. The official spoke on condition of anonymity.
U.S. intelligence did intercept communications discussing the attack between officials in central command and in the field. But these do not clearly implicate Assad or his entourage in ordering the use of chemicals, sources familiar with the material said.
While U.S. experts say the most likely chemical agent used in the attack was the nerve gas sarin, scientific evidence proving this still remains incomplete, one of the sources added.
(Editing by Warren Strobel and David Storey)