WASHINGTON (AP) — The Obama administration is opposed to even limited U.S. military intervention in Syria because it believes rebels fighting the Assad regime wouldn't support American interests if they were to seize power right now, according to the chairman of the Joint Chiefs of Staff.

Effectively ruling out U.S. cruise missile attacks and other options that wouldn't require U.S. troops on the ground, Gen. Martin Dempsey said in a letter to a congressman that the military is clearly capable of taking out Syrian President Bashar Assad's air force and shifting the balance of the Arab country's 2½-year war back toward the armed opposition.

But he said such an approach would plunge the United States deep into another war in the Arab world and offer no strategy for peace in a nation plagued by ethnic rivalries.

"Syria today is not about choosing between two sides but rather about choosing one among many sides," Dempsey said in the letter Aug. 19 to Rep. Eliot Engel, D-N.Y. "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."

Dempsey's pessimistic assessment will hardly please members of the fractured Syrian opposition leadership and some members of the administration who have championed greater support to help the rebellion end Assad's four-decade family dynasty. Despite almost incessant bickering and internal disputes, some opposition groups have worked with the United States and other European and Arab supporters to try to form a cohesive, inclusive movement dedicated to a democratic and multiethnic state.

But those fighting the Assad government range wildly in political beliefs and not all are interested in Western support.

As the conflict has gone on, killing more than 100,000 people and ripping apart the delicate sectarian fabric of Syrian society, 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 regime has committed. 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.

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Dempsey said Syria's war was "tragic and complex."

"It is a deeply rooted, long-term conflict among multiple factions, and violent struggles for power will continue after Assad's rule ends," he wrote. "We should evaluate the effectiveness of limited military options in this context."

On Wednesday, two Syrian pro-opposition groups claimed government forces carried out a toxic gas attack that killed at least 100 people near the capital, Damascus. The Syrian government denied the claims. The reported death toll would make it the deadliest alleged chemical attack in Syria's civil war.

Despite calling for Assad to leave power in 2011, President Barack Obama has steadfastly refused to allow the U.S. to be drawn directly into the conflict. Officials have said for the past couple of months, however, that the U.S. is prepared to provide lethal aid to vetted, moderate units among the opposition ranks. It's unclear what, if any, weapons have been delivered so far.

Dempsey's letter to Engel was another follow-up to a sharp examination he faced in July from the Senate Armed Services Committee ahead of a reconfirmation vote. Unable to answer questions by Republican Sen. John McCain of Arizona and Democratic Sen. Carl Levin of Michigan, 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 at a cost as much as $1 billion a month and with no assurance that it would change the war's momentum.

He also discouraged options such as training vetted rebel groups, limited strikes on Syria's air defenses and creating a buffer zone for the opposition, stressing the need to avoid an outcome similar to Iraq or Afghanistan by preserving a functioning state for any future power transfer. And he cited risks such as lost U.S. aircraft.

Engel, another advocate of more forceful U.S. action, joined the debate by proposing the use of cruise missiles and other weapons against Syrian government-controlled air bases in an Aug. 5 letter to Dempsey. The congressman said such strikes would ground Assad's air force and reduce the flow of weapons to his government from Iran and Russia, while costing less to U.S. taxpayers and requiring no American troops on the ground in Syria or in its airspace.

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

"We can destroy the Syrian air force," he said. "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. Stated another way, it would not be militarily decisive, but it would commit us decisively to the conflict."

"The use of U.S. military force can change the military balance," Dempsey added. "But it cannot resolve the underlying and historic ethnic, religious and tribal issues that are fueling this 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.

Engel responded with disappointment Wednesday.

"Until we are prepared to severely diminish the regime's ability to inflict harm upon its own citizens and even the playing field — such a moderate opposition stands little chance against the regime's scuds, tanks and planes," he said in a statement.