WASHINGTON >> The Obama administration searched for answers today about a reported chemical weapons attack in Syria that would mark the most flagrant violation yet of the U.S. “red line” for potential military action. But the possibility of intervention seemed ever smaller after America’s top general offered a starkly pessimistic assessment of options.

Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, said in a letter this week to a congressman that the administration is opposed to even limited action in Syria because it believes rebels fighting the Assad government wouldn’t support American interests if they seized power.

Dempsey said the U.S. military is clearly capable of taking out Assad’s air force and shifting the balance of the war toward the armed opposition. But such an approach would plunge the U.S. into the war without offering any strategy for ending what has become a sectarian fight, he said.

Today, Syrian anti-Assad activists accused the government of carrying out a toxic gas attack in the eastern suburbs of Damascus, killing at least 100 people including children. The claims coincided with a visit by a U.N. chemical weapons team to three previous sites of alleged attacks. Syrian President Bashar Assad’s government rejected the accusations, and U.S. officials said they were seeking details of what happened.

For the United States, the death toll and painful images again put a spotlight on President Barack Obama’s pledge almost exactly a year ago to respond forcefully to any chemical weapons use by the Assad government. Since then, the administration has said it has confirmed that Syrian forces have committed such attacks, and the U.S. has ordered a lethal aid package of small arms to be sent to some rebel groups, though it’s unclear what if any weapons have been delivered.

Yet up to now, Obama has refused all options of direct U.S. military intervention in a civil war that has killed more than 100,000 people and displaced millions. 

“The United States is deeply concerned by reports that hundreds of Syrian civilians have been killed in an attack by Syrian government forces, including by the use of chemical weapons,” White House spokesman Josh Earnest said today. 

“We are working urgently to gather additional information,” Earnest said, adding that Washington has asked for U.N. investigators to be granted access to the area of the fighting. He made no mention of possible consequences if chemical weapons use is confirmed.

Obama has stated that he doesn’t want to be drawn into another Mideast conflict after a decade of fighting in Iraq and Afghanistan, and polling suggests he has the public’s support on that.

Dempsey, in his letter, said, “Syria today is not about choosing between two sides but rather about choosing one among many sides,” In the Aug. 19 letter to Rep. Eliot Engel, D-N.Y., he said, “It is my belief that the side we choose must be ready to promote their interests and ours when the balance shifts in their favor. Today, they are not.”

Despite Dempsey’s assessment of the forces fighting Assad, Obama recognized the Syrian opposition coalition as “the legitimate representative” of the Syrian people more than eight months ago. And Secretary of State John Kerry has repeatedly backed the moderate vision promoted by Salim Idris, the rebel military chief.

But the more than 50 distinct rebel groups fighting to end the Assad family’s four-decade dynasty range wildly in political beliefs and not all are interested in Western support.

As the conflict has dragged on, al-Qaida-linked rebels and other extremist groups have been responsible for some of the same types of massacres and ethnic attacks that the Assad government is accused of. On Tuesday, Kurdish militias battled against al-Qaida-linked fighters in the northeast in fighting that has fueled a mass exodus of refugees into Iraq and risks exploding into a full-blown side conflict.

Dempsey said Syria was experiencing “a deeply rooted, long-term conflict among multiple factions and violent struggles for power” that will continue after Assad’s rule ends.

His letter to Engel was a follow-up to the sharp examination he faced in July from the Senate Armed Services Committee as part of his reconfirmation.  

Dempsey sent a letter afterward saying the establishment of a no-fly zone to protect the Syrian rebels would require hundreds of U.S. aircraft, cost as much as $1 billion a month and offer no assurance of changing the war’s momentum. He also discouraged training of rebel groups, limited strikes on Syria’s air defenses or creating a buffer zone for the opposition, citing the high costs involved and risks such as lost U.S. aircraft.

Engel, an advocate of more forceful U.S. action, proposed the use of cruise missiles and other weapons against Syrian government-controlled air bases in an Aug. 5 letter to Dempsey. The congressman said strikes could ground Assad’s air force and reduce weapons flow to his government from Iran and Russia, while costing less to U.S. taxpayers and requiring no American troops in Syrian territory or airspace.

Dempsey said this approach wouldn’t tip the balance against Assad and wouldn’t solve the deeper problems plaguing Syria.

“The loss of Assad’s air force would negate his ability to attack opposition forces from the air, but it would also escalate and potentially further commit the United States to the conflict,” Dempsey said. “It would not be militarily decisive, but it would commit us decisively to the conflict.”

Instead, he spoke in favor of an expansion of the Obama administration’s current policy.

The U.S. can provide far greater humanitarian assistance and, if asked, do more to bolster a moderate opposition in Syria. Such an approach “represents the best framework for an effective U.S. strategy toward Syria,” Dempsey said.