By Phil Stewart
WASHINGTON, Aug 21 (Reuters) - The U.S. military could - if asked - significantly boost American support to Syria's moderate opposition, the top U.S. military officer said, suggesting moderates were still unable to shape events should President Bashar al-Assad fall from power.
General Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, made the comments in a letter to a congressman seen by Reuters on Wednesday, in which he renewed his concerns about the limits of U.S. military power to steer events in Syria's war.
"We could, if asked to do so, significantly increase our effort to develop a moderate opposition," he wrote, without offering details.
However, Dempsey added in the response to a query from Eliot Engel, the top Democrat on the House Foreign Affairs Committee: "Syria today is not about choosing between two sides but rather about choosing one among many sides."
"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," Dempsey wrote. "Today, they are not."
The United States is providing non-lethal support to the opposition, but has been slow to make good on promises to deliver weapons to moderates.
Dempsey said the U.S. military could destroy Syria's air forces to impose a cost for Assad's "unacceptable behavior," and address on a far greater scale the humanitarian crisis stemming from Syria's conflict.
But he stressed that U.S. intervention would not be militarily decisive and would not extinguish the religious, tribal and ethnic tensions fueling the conflict.
Dempsey's letter came after a trip he made to Jordan and Israel that underscored the complexity of the conflict, in which Syria is becoming a magnet for foreign Sunni Muslim fighters seeking to join what they see as a holy war against Shi'ites.
Some in the U.S. Congress, including Engel, believe President Barack Obama must take more aggressive military steps in Syria, particular following reports of use of chemical weapons by Assad's forces.
Syria's opposition accused Assad's forces on Wednesday of gassing many hundreds of people - by one report as many as 1,300 - in what would, if confirmed, be the world's worst chemical weapons attack in decades.
"I reject the notion that our involvement in Syria would simply constitute 'choosing sides' between one armed group and another," Engle said in a statement, responding to Dempsey's letter.
"Rather, our involvement represents a choice between hastening the end of the Assad regime or continuing to allow the cycle of violence, displacement, and terror to continue unabated."
In comments to reporters after meeting leaders in Jordan and Israel last week, Dempsey did not signal interest from those allies for overt U.S. military action in Syria.
However, Amman asked Dempsey for manned surveillance aircraft to help keep watch on its long border with Syria, a request which follows U.S. moves to station F-16 fighter jets and Patriot missiles in Jordan. Dempsey said he would take that request back to Washington for consideration. (Reporting by Phil Stewart; Editing by David Brunnstrom)