U.S. military options in Syria remain challenging

(Reuters) - The Obama administration is promising to step up assistance to Syrian rebels after concluding Damascus used chemical weapons against opponents of President Bashar al-Assad, but the military options facing the United States are no easier than before.

President Barack Obama has been slow to move toward military assistance for Syria's opposition in the past and is likely to continue to move judiciously, looking to work with allies in any intervention in the country's civil war.

"Thankfully the president is being cautious about his military options. This is not a place where America can go it alone," said David Solimini, a vice president at the Truman National Security Project think tank.

The Obama administration's possible military choices range from limited one-off missile strikes from ships - one of the less-complicated scenarios - to bolder operations like carving out no-fly safe zones.

White House officials on Thursday played down the prospects of a no-fly zone, citing the open-ended costs and uncertainty of such an operation in a place like Syria, with its well-defended air space. But they did not rule it out either.

The White House did, however, rule out the more politically unpalatable possibility of sending U.S. forces into the country.

A more likely move would be to begin providing lethal assistance to Syrian rebels. The White House said on Thursday it would increase support to the opposition Supreme Military Council, including military support, but it did not specify lethal aid.

A source close to the matter said on Thursday that Obama has authorized sending some U.S. weapons to Syrian rebels as part of a new package of military support. There was no word on the type of weaponry the United States would provide or when it would be delivered.

Obama had opposed limited steps like arming anti-government rebels, but pressure to deepen U.S. involvement in Syria's civil war after American intelligence agencies concluded Damascus used chemical weapons appears to have changed the calculation.

Obama last August pledged he would consider more significant aid to the rebels if Assad's regime crossed a "red line" by moving around or using sizeable amounts of chemical weapons.

Representative Ed Royce, chairman of the House of Representatives Foreign Affairs Committee, said on Thursday: "It is now clear that the Assad regime has crossed a red line. ... I encourage the administration to begin in earnest arming the Free Syrian Army."

STRIKES, NO-FLY ZONE

One form of military intervention that could limit to some extent U.S. and allied involvement in Syria's war would be one-off strikes on pro-Assad forces or infrastructure tied to chemical weapons use. Given Syria's air defenses, planners may choose to fire missiles from ships at sea.

"The most proportional response (to limited chemical weapons use) would be a strike on the units responsible, whether artillery or airfields," Jeffrey White, a Middle East expert at the Washington Institute For Near East Policy, said recently.

"It would demonstrate to Assad that there is a cost to using these weapons. The problem so far is that there's been no cost to the regime from their actions."

It is not clear how the Syrian government would respond and if it would try to retaliate militarily against the U.S. forces in the region. U.S. military involvement would also upset Russia, which has a naval facility on Syria's Mediterranean coast.

Another option the Pentagon has examined involves the creation of humanitarian safe areas in support of Turkey and Jordan that would also be no-fly zones off limits to the Syrian air force - an option favored by lawmakers including Republican U.S. Senator John McCain.

That would involve taking down Syrian air defenses and destroying Syrian artillery from a certain distance beyond those zones, to protect them from incoming fire.

But officials, including Defense Secretary Chuck Hagel, have warned that establishing safe zones would tie the United States more closely to Syria's messy conflict. Assad would almost certainly react.

The U.S. military has also completed planning for going into Syria and securing its chemical weapons under different scenarios, including one in which Assad falls from power and his forces disintegrate, leaving weapons sites vulnerable to pillaging.

But any eventual move to put U.S. boots on the ground in Syria remains a remote possibility.