By Phil Stewart

WASHINGTON (Reuters) - The United States has a less clear understanding of Syria's opposition than it did last year, the top U.S. military officer said on Monday, in comments likely to disappoint rebels hoping that America might be inching toward a decision to arm them.

"About six months ago, we had a very opaque understanding of the opposition and now I would say it's even more opaque," said General Martin Dempsey, chairman of the U.S. military's Joint Chiefs of Staff.

Dempsey, who is President Barack Obama's top uniformed military adviser, said he would also advise extreme caution when deliberating any military options in Syria - saying the conflict posed "the most complex set of issues that anyone could ever conceive, literally."

"I don't think at this point I can see a military option that would create an understandable outcome," Dempsey told the Center for Strategic and International Studies, a Washington think tank. "And until I do, it would be my advice to proceed cautiously."

More than 70,000 Syrians have been killed in a fierce conflict that began with peaceful protests against the government of President Bashar al-Assad two years ago. Some 860,000 Syrians have fled abroad and several million are displaced within the country or need humanitarian assistance.

In a sign of the complexities of the conflict, Syrian government aircraft fired rockets into northern Lebanon, in what the U.S. State Department described on Monday as a "significant escalation." Rebels said they had fired mortar bombs at the presidential palace in Damascus.

Dempsey along with former Defense Secretary Leon Panetta and the heads of the CIA and State Department favored the idea of arming Syrian rebels during discussions within the Obama administration last year.

He told reporters last month he thought it might help end the crisis more quickly and avert the collapse of government institutions.

Still, there was never a specific plan under review and Obama decided against that option. U.S. officials have voiced concerns that any weapons provided by the United States could fall into the wrong hands.

Dempsey, asked whether he would consider more robust support for Syrians that stopped short of direct U.S. military intervention, acknowledged there were "opportunities" in Syria but suggested any actions would be led by U.S. allies.

"We very much do believe that the answer to Syria is through partners, because ... there's a greater likelihood that they'll understand the complexities than we would," he said.

(Editing by Warren Strobel and Paul Simao)