The US navy is expanding its presence in the Mediterranean Sea with a fourth cruise-missile-armed warship, amid indications from Chuck Hagel, the defence secretary, that the US is positioning assets in anticipation of any decision by President Barack Obama to order military action on Syria.

Friday's decision came after rebels and activists claimed that a chemical-weapons attack on a rebel-held area in the outskirts of the Syrian capital Damascus killed hundreds of people.

Barack Obama, the US president, has called the incident "a big event of grave concern".

His security advisers are expected to convene at the White House this weekend to discuss US options regarding Syria.

The USS Mahan, a cruise-missile armed warship, had finished its deployment and was due to head back to its home base in Norfolk, Virginia, but the commander of the US Sixth Fleet decided to keep the ship in the region, Reuters news agency and CNN reported.

The USS Ramage arrived to replace the USS Mahan, but the Mahan is going to stay around a bit longer, so temporarily there will be four, CNN said.

An official, who was not authorised to speak publicly, told Reuters that the navy had received no orders to prepare for any military operations regarding Syria.

Options on Syria

In a statement afterwards, Hagel, who was travelling to Malaysia, discussed the decision to keep the Mahan in the Mediterranean.

He said Obama had asked the Defence Department for options on Syria.

"The Defence Department has responsibility to provide the president with options for all contingencies," he said. "And that requires positioning our forces, positioning our assets, to be able to carry out different options - whatever options the president might choose."

Asked whether it was fair to report that the US had moved assets, Hagel said: "I don't think I said that. I said that we are always having to prepare - as we give the president options - prepare our assets and where they are and the capability of those assets to carry out the contingencies we give the president."

Meanwhile, Obama's security advisers are planning to convene at the White House this weekend to discuss US options, including possible military action.

Speaking to Reuters on condition of anonymity, an official cautioned against expecting that any final decision might come out of this next round of discussions.

A UN team is in the country to investigate the mutual allegations of the rebels and the Syrian regime on chemical weapons use in Syria.

Footage shot by an independent journalist for Britain's ITV News appears to show victims of the alleged chemical attack in question.