The Obama administration searched for answers Wednesday 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.

On Wednesday, 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. Syrian President Bashar Assad’s government rejected the accusations.

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 Wednesday. “We are working urgently to gather additional information.”

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.

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.