A punitive salvo of US cruise missiles of the kind being contemplated in Washington will do little to bring Bashar al-Assad's chemical-armed Syrian regime to heel, experts warn.

US warships are on standby in the eastern Mediterranean to unleash a barrage of guided weapons if President Barack Obama decides to respond to Assad's alleged use of chemical weapons against Syrian civilians.

The White House says no final decision has been made, but on Monday US Secretary of State John Kerry set the stage for military action.

"Make no mistake," Kerry said. "President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people."

Kerry's language was tough but, separately, military officials -- speaking on condition of anonymity -- played down the scope of the military action that is envisaged.

There will be no ground invasion of the type launched against Iraq in 2003, nor even a months-long NATO-led bombing campaign like those over Serbia in 1999 or Libya in 2011.

Instead, at some point in the coming days, Assad will wake up to a powerful but brief onslaught of Tomahawk cruise missiles fired from US Navy destroyers.

The goal of the attacks, which once it starts could be over within days, would not be to tip the balance in Syria's three-year-old civil war, nor to topple Assad.

Instead, US officials said, it would "send a message" that using chemical weapons is unacceptable and degrade Assad's will and capability to repeat last week's alleged large-scale gas attack.

But independent observers doubt that such a limited military intervention would be effective.

"It has to be large enough and to inflict enough pain and cost to the Syrians so that they would be discouraged from using chemical weapons again," warned Richard Haas of leading think tank the Council on Foreign Relations.

Christopher Harmer, a former naval officer who when in service helped develop the US Navy's plans for cruise missile use, said such a barrage would inflict only a temporary setback to Assad.

"Each of the four US Navy surface combatants currently in the eastern Mediterranean would have approximately 45 TLAM onboard, for a total of 180 TLAM," he wrote.

These TLAMs -- Tomahawk Land Attack Missiles -- pack a punch but "cannot eliminate the regime's military or chemical weapons capabilities ... nor cause more than a temporary degradation in regime operations."

"Such a strike will be ineffective unless it is part of a coherent, properly resourced effort towards achieving clearly-articulated US strategic aims in Syria," Harmer warned.

"The fall of the Assad regime is one clear objective. Depriving Assad of the ability to use or proliferate chemical weapons is another. Punishing Assad for using chemical weapons is not."

And some analysts warn that such a strike would not just be ineffective, it might be counterproductive.

Robert Satloff, of the Washington Institute for Near East Policy, said it would be mistake to cede to the temptation of a "limited punitive response."

"Is American military action designed to punish Assad for violating the international norm on chemical weapons?" he asked, in an op-ed for Washington news outlet Politico.

"If so, it will merely have the effect of defining for Assad the acceptable tools for mass killing - perhaps only the acceptable quantities of CW to use at any given time - and will have little impact on the outcome of the Syrian conflict; in fact, it might just embolden Assad and his allies."

The tougher US stance comes more than a year after Obama warned that any use of chemical weapons by Assad's forces would cross a "red line" and trigger a US response, and two months after Washington said he had done so.

There is, however, little historical evidence that punitive strikes work. Former Libyan strongman Moamer Kadhafi's regime was targeted by one night of strikes in 1986, but bombed a US airliner two years later.

Antony Cordesman, a fellow of the Center for International and Strategic Studies (CSIS), said the short-term plan appeared to miss the bigger picture.

"The key challenge in Syria is scarcely to end the use of chemical weapons," he wrote.

"The real challenge is some 120,000 dead, another 200,000-plus wounded, and as many as 20 percent of its 22.5 million people have been displaced inside the country or are living outside it as refugees"

"There is no point in fighting a war against chemical weapons. There is no point in US military symbolism or massive unilateral military action. There is a point in trying to use force to end the suffering, the fighting, and repression- and serve our national interest while we meet the needs of the Syrian people and our allies."