The likely use of chemical weapons in Syria ramps up pressure on US President Barack Obama to act, but any military options are limited and risky for a leader haunted by Iraq.

The White House has made clear that it is not rushing into the conflict and stopped short of saying that Syrian President Bashar al-Assad had crossed what Obama famously called the "red line" of chemical weapons.

Obama repeated Friday that chemical weapons would be a "game changer" but called for a vigorous investigation after the United States, Israel and Britain all cited signs that the regime attacked with the deadly agent sarin.

"We have seen very bad movies before when intelligence is perceived to have driven policy decisions that in the full light of day have proven wrong," a US defense official said on condition of anonymity.

The official was referring to assertions -- later proven false -- by former president George W. Bush's administration of weapons of mass destruction in Iraq, the justification for a 2003 invasion that brought eight years of war that killed nearly 4,500 US soldiers and tens of thousands of Iraqis.

Andrew Tabler of the Washington Institute for Near East Policy said that the use of chemical weapons would show that Assad was testing US red lines and trying to frighten the rebels into surrendering in the two-year civil war.

"The problem is that American credibility is on the line," Tabler said. "We're at a turning point. How this crisis unfolds will largely be determined by our response."

White House spokesman Jay Carney said that "all options are on the table," which "include but are not exclusive" to military force.

Secretary of State John Kerry recently announced the United States would double aid to the Syrian opposition to $250 million. But the United States says it only provides "non-lethal" assistance, amid persistent concerns about the nature of the rebels in a conflict that has left more than 70,000 dead.

The Sunni Muslim rebels are believed to receive military support from Gulf Arab monarchies Qatar and Saudi Arabia, while Assad -- a member of the minority Alawite sect -- is allied with Iran and Russia.

Nancy Pelosi, the leader of Obama's Democratic Party in the House of Representatives and a staunch opponent of the Iraq war, said that non-lethal aid needed to be taken "to the next step" -- but opposed deployment of US troops.

No one expects a full-out military invasion of Syria from a nation weary after Iraq and Afghanistan, from where Obama plans to pull out the last combat troops next year.

The United States has stationed 250 troops, mostly special forces, in the desert of neighboring Jordan to train local soldiers and potentially prepare for a last-resort mission to secure Syria's chemical weapons.

But General Martin Dempsey, the chairman of the Joint Chiefs of Staff, has expressed reservations over the ability of the United States to secure Syrian chemical weapons, estimated in the hundreds of tons.

The top US military officer, testifying to the Senate Armed Services Committee on April 17, said that seizing weapons was complicated "simply because they've been moving it and the number of sites is quite numerous."

Danielle Pletka of the conservative American Enterprise Institute said that the United States should begin the process of arming "moderates" among the Syrian rebels, a step supported by Britain and France.

Pletka said that the United States could also cripple Syria's air power and create a "humanitarian corridor" in the country.

"These are doable goals, requiring no boots on the ground," she said.

But Representative Brad Sherman, a Democrat on the House Foreign Affairs Committee, said that a no-fly zone was not a "no-casualties option."

In 2011, Western forces had to bomb Libya's anti-air defenses to create a no-fly zone in support of rebels who toppled dictator Moamer Kadhafi.

Before he stepped down, former secretary of defense Leon Panetta said last year that a similar effort in Syria would be "100 times worse than what we dealt with in Libya."

Syria's air power dates from the Soviet era but is still significant. According to the Institute for the Study of War, Syria has about 650 static and 300 mobile air defense sites, along with around 150 air-worthy warplanes.

Israel launched a covert attack in 2007 that destroyed a purported Syrian nuclear reactor. But air attacks on chemical weapons sites would be highly risky as toxic materials could be emitted into the environment.

"The easiest move by the administration would be to pick out a valuable, discrete target of the regime's and obliterate it -- with cruise missiles and possibly manned aircraft as well," said Kenneth Pollack of the Brookings Institution.

"The strike would be a warning to the Syrians that worse would follow if the regime does not desist from further chemical warfare use and a demonstration that Washington will back up its red line."