"The intelligence that we have raises serious concern that this is being considered," he told reporters. Other administration officials in recent days have spoken about Syrians preparing weapon components of sarin gas. The new activity, coupled with fears that rebel advances are making Assad more desperate, have led to the fear that he is deploying the weapons.
On Capitol Hill, some senators even suggested military action against Assad.
Sen. John McCain told reporters the U.S. should "be ready to do whatever is necessary to prevent" the use of chemical weapons, "including the option of military intervention."
Syria's Deputy Foreign Minister Faisal Mekdad on Thursday accused the United States and Europe of using the issue of chemical weapons to justify a future military intervention against Syria. He warned that any such intervention would be "catastrophic."
Brahimi is hoping to resuscitate something akin to the plan crafted earlier this year by his predecessor as Syria peace envoy, former U.N. Secretary-General Kofi Annan. Annan's plan never got off the ground, and he resigned his post in frustration.
Annan's plan demanded several steps by the Assad regime to de-escalate tensions and end the violence that activists say has killed more than 40,000 people since March 2011. It then required Syria's opposition and the regime to put forward candidates for a transitional government, with each side having the right to veto nominees proposed by the other.
If anything resembling that plan progresses, it would surely mean the end of more than four decades of an Assad family member at Syria's helm. The opposition has demanded Assad's departure and has rejected any talk of him staying in power. Yet it also would grant regime representatives the opportunity to block Sunni extremists and others in the opposition that they reject.
The United States blamed the collapse on Russia for vetoing a third resolution at the U.N. Security Council that would have applied world sanctions against Assad's government for failing to live by the deal's provisions.
Russia insisted that the Americans unfairly sought Assad's departure as a precondition and worried about opening the door to military action, even as Washington offered to include language in any U.N. resolution that would have expressly forbidden outside armed intervention.
The same pitfalls threaten Brahimi's process. The Obama administration is likely to insist anew that it be internationally enforceable — a step Moscow may still be reluctant to commit to.
But with the war turning against Assad, U.S. officials are hoping that Russia will be prepared to drop its support for him.
In Syria, government forces on Thursday shelled rebellious suburbs around Damascus and clashed with rebels in the capital itself and in Aleppo, Syria's largest city.
Robert Ford, the U.S. ambassador in Damascus until his recall earlier this year, pointed to the closure of Syria's main airport and the rebel capture of defense sites inside the Damascus beltway as key indicators of the conflict's direction. "The writing is on the wall," he said at the Foundation for Defense of Democracies forum in Washington.
Also Thursday, NATO advanced its plan to place Patriot missiles and troops along Syria's border with Turkey to protect against potential attacks. Assad's regime blasted the move as "psychological warfare," saying the new deployment would not deter it from seeking victory over rebels it views as terrorists.
Associated Press writers Shawn Pogatchnik in Dublin and Pauline Jelinek and Kimberly Dozier in Washington contributed to this report.
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