By Phil Stewart and Peter Apps
WASHINGTON (Reuters) - Despite President Barack Obama's pledge that Syria's use of chemical weapons is a "game changer" for the United States, he is unlikely to turn to military options quickly and would want allies joining him in any intervention.
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.
One of the most politically unpalatable possibilities envisions sending tens of thousands of U.S. forces to help secure Syrian chemical weapons.
Obama has so far opposed limited steps, like arming anti-government rebels, but pressure to deepen U.S. involvement in Syria's civil war has grown since Thursday's White House announcement that President Bashar al-Assad likely used chemical weapons.
After fighting wars in Afghanistan and Iraq, the Pentagon is wary of U.S. involvement in Syria. The president's top uniformed military adviser, General Martin Dempsey, said last month he could not see a U.S. military option with an "understandable outcome" there.
"There's a lot of analysis to be done before reaching any major decisions that would push U.S. policy more in the direction of military options," a senior U.S. official told Reuters.
That caution is understandable, given the experience of Iraq where the United States went to war based on bad intelligence about weapons of mass destruction. The Pentagon has made repeated warnings of the enormous risks and limitations of using American military might in Syria's civil war.
STRIKES, NO-FLY ZONE
One form of military intervention that could to some extent limit 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," said Jeffrey White, a former senior official at the Pentagon's Defense Intelligence Agency and a Middle East expert who is now a defense fellow at the Washington Institute For Near East Policy.
"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 that the Pentagon has examined involves the creation, ostensibly in support of Turkey and Jordan, of humanitarian safe areas that would also be no-fly zones off limits to the Syrian air force - an option favored by lawmakers including Senator John McCain of Arizona.
This would involve taking down Syrian air defenses and destroying Syrian artillery from a certain distance beyond those zones, to protect them from incoming fire.
Advocates, including in Congress, say a safe zone inside Syria along the Turkish border, for example, would give needed space for rebels and allow the West to increase support for those anti-Assad forces it can vet.
Still, as officials, including Defense Secretary Chuck Hagel, have warned, once established, a safe zone would tie the United States more closely to Syria's messy conflict. Assad would almost certainly react.
"Once you set up a military no-fly zone or safe zone, you're on a slippery slope, mission creep and before you know it, you have boots on the ground," said Bruce Riedel, a former CIA analyst and Middle East expert at the Brookings Institution.
"Or you end up like Libya where you don't really have a control mechanism for the end-game, should you end up with chaos."
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.
The U.S. fears anti-Assad Islamist rebels affiliated to al Qaeda could grab the chemical weapons but a U.S. intervention into Syria to get the arms would require tens of thousands of American troops.
Asked if he was confident the U.S. military could secure Syria's chemical weapons stock, Dempsey told Congress: "Not as I sit here today simply because they have been moving it and the number of sites is quite numerous."
IS THERE A WILLING COALITION?
Obama said on Friday that he would seek to mobilize the international community around Syria, as he attempts to determine whether pro-Assad forces used chemical weapons.
British and French officials have long made it clear their countries might be willing to join in any U.S.-led action under the right circumstances.
But Hagel warned last week that "no international or regional consensus on supporting armed intervention now exists." Once a fervent advocate of foreign intervention in Syria, Turkey has grown frustrated with the fractured opposition to Assad and with international disunity.
NATO chief Anders Fogh Rasmussen has ruled out Western military intervention and U.S. Admiral James Stavridis, NATO's supreme allied commander, cautioned last month that the alliance would need agreement in the region and among NATO members as well as a U.N. Security Council resolution - something that looks unlikely given probable opposition from Russia and China.
The Pentagon has focused over the past year on synchronizing defense planning on Syria, including with Britain, France and Canada.
It is also enhancing its military presence in Jordan by ordering some 200 Army planners into Jordan to focus on Syria scenarios. That would be a better group to coordinate any military or humanitarian action than the ad-hoc U.S. military team previously in Jordan.
Obama met Jordan's King Abdullah at the White House on Friday and Hagel traveled to Jordan this week, as well as to Israel, Saudi Arabia, Egypt and the United Arab Emirates.
"It seems increasingly clear that the Obama administration is feeling pressure to act," said Mona Yacoubian, a former State Department official and now a Syria expert at the Stimson Center in Washington.
"But they will likely seek two things: conclusive evidence and multilateral support/participation in whatever action (they) choose, which I think would be limited, targeted air strike."
(Editing by Alistair Bell and Sandra Maler)