U.S. Defense Secretary Chuck Hagel on Friday strongly suggested the Pentagon was moving forces into place ahead of possible military action against Syria, even as President Barack Obama voiced caution.

Obama has said Washington must be wary of costly and difficult foreign interventions, as calls mount for action against the regime of Syrian President Bashar al-Assad over alleged chemical warfare.

U.S. commanders have nevertheless prepared a range of "options" for Obama if he chooses to proceed with military strikes against Damascus, Hagel told reporters before landing in Kuala Lumpur.

"The Defense Department has a responsibility to provide the president with options for all contingencies," Hagel said.

"And that requires positioning our forces, positioning our assets to be able to carry out different options — whatever the president might choose."

But Hagel declined to provide any details on the deployment of U.S. ships, aircraft or troops, as the Obama administration reportedly contemplated cruise missile strikes against Assad's forces.

Hagel's comments came as a defense official said the U.S. Navy would expand its presence in the Mediterranean with a fourth warship armed with cruise missiles.

The U.S. Sixth Fleet, with responsibility in the Mediterranean, has decided to keep the USS Mahan in the region instead of letting it return to its home port in Norfolk, Virginia.

Three other destroyers are currently deployed in the area — the USS Gravely, the USS Barry and the USS Ramage. All four warships are equipped with several dozen Tomahawk cruise missiles.

The reinforcement would allow the Pentagon to act more rapidly if Obama were to order a military strike.

"The president has asked the Defense Department for options. Like always, the Defense Department is prepared and has been prepared to provide all options for all contingencies to the president of the United States," Hagel said.

The Pentagon chief made clear that no decision had been taken on whether to employ military force as the more than two-year-old conflict rages on.

American newspapers have suggested disagreements within the administration over the risks of another U.S. military intervention in the Middle East.

In an interview aired earlier Friday on CNN, Obama voiced caution.

He said Syrian opposition allegations that hundreds of people had been killed in a gas attack near Damascus this week were more serious than previous charges against Assad's regime.

"What we've seen indicates clearly this is a big event, of grave concern," Obama said.

One year after warning that the use of chemical arms in the vicious Syrian conflict would cross a US "red line", Obama said Americans expect him to protect their long-term national security interests — but avoid foreign entanglements.

"Sometimes what we've seen is folks will call for immediate action, jumping into stuff that does not turn out well, gets us mired in very difficult situations," Obama said.

He warned that America could get "drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region."

The president also said that there were questions about whether the United States would violate international law if it attacked another country without a United Nations Security Council mandate.

And, after ending the Iraq war and as he brings troops home from Afghanistan, Obama noted the cost in U.S. lives and financial resources of foreign military action.

Obama observed that the latest attack was conducted on a much wider scale than a previous one in Syria that the United States deemed to have been the result of chemical weapons.

On that occasion, Obama decided for the first time to send direct military aid to vetted Syrian rebels, though has declined to specify exactly what help Washington is providing.

Syria has vigorously denied its forces were guilty of a chemical attack on the rebel-held area.

Hagel, who began a week-long tour of Southeast Asia, said he expected American intelligence agencies to "swiftly" assess whether Damascus was to blame.

He warned that if the Assad regime had resorted to chemical weapons, "there may be another attack coming."

The U.S. government would not rule out unilateral action, Hagel said, but he stressed the need to work with international "partners."

"If the intelligence and facts bear out what appears to be what happened -- use of chemical weapons — then that is not just a United States issues, it's an international issue," he said.

"It violates every standard of international behavior."

Before the alleged chemical weapons assault, the U.S. military's top officer, Gen. Martin Dempsey, had expressed caution over any military action in Syria.

Dempsey had warned that imposing a no-fly zone would carry the risk of dragging the US into a protracted conflict or inadvertently aiding Islamist militants fighting Damascus.

Asked about Dempsey's statements, Hagel said he agreed with the four-star general's assessment, calling it "very accurate."

National Security Adviser Susan Rice took to Twitter to urge the Syrian government to permit U.N. inspectors to probe the latest chemical weapons allegations.

"What is Bashar al-Assad hiding? The world is demanding an independent investigation of Wednesday's apparent CW attack. Immediately," she wrote.

"Otherwise, we'll all conclude that Assad is guilty and lying — again."