The U.S. moved warships closer to the eastern Mediterranean on Saturday, as U.S. President Barack Obama discussed next steps in a possible military intervention in Syria with British Prime Minister David Cameron.

The White House said the phone conversation was the first with a foreign leader since reports this week that hundreds of Syrians had been killed by the alleged chemical attack in a suburb of Damascus, the capital.

In separate phone conversations, Canadian Prime Minister Stephen Harper also conferred on Saturday with Cameron and French president François Hollande about an "appropriate response" to the Syria crisis, according to officials in Ottawa.

Cameron reportedly wants to use a "game-changing" resolution at the United Nations to set an ultimatum for Assad to disarm.

According to Britain's Telegraph newspaper, the draft text is being modelled after the 2002 UN resolution that went after Saddam Hussein, offering the late Iraqi dictator one last chance to disarm or else face "serious consequences."

The discussions among world leaders come amid international calls for foreign military intervention in Syria.

Meeting on the issue Saturday with his national security team, Obama received a detailed review of the range of options he has requested for the U.S. and its international partners to respond if the fact-finding process concludes that Syrian President Bashar Assad engaged in deadly chemical warfare, the White House said.

At the same time, Obama has emphasized that quick intervention in the years-old Syrian civil war was problematic because of the international considerations that should precede a military strike.

Obama discussed the situation in Syria by telephone with British Prime Minister David Cameron, the White House said. It was Obama's first known conversation with a foreign leader about Syria since the reports this week that hundreds of Syrians had been killed by the alleged chemical attack in a suburb of Damascus, the capital.

UN High Representative for Disarmament Affairs Angela Kane (second from left) arrived in Syria on Saturday to push for access to a suspected chemical weapons attack site for UN inspectors, who are already in Syria to investigate previous accusations. (Khaled al-Hariri/Reuters)

The White House said the two leaders expressed "grave concern" about the reported chemical weapons use, which both of their countries oppose.

A statement from Cameron's office at No. 10 Downing St. said the prime minister and Obama "reiterated that significant use of chemical weapons would merit a serious response from the international community."

The Syrian government denies the claims. It also has warned the U.S. against taking military action, saying such a step would set the Middle East ablaze.

Continuing to gather facts

Officials have said Obama will decide how to respond once the facts are known.

"In co-ordination with international partners and mindful of the dozens of contemporaneous witness accounts and record of the symptoms of those killed, the U.S. intelligence community continues to gather facts to ascertain what occurred," the White House said after Obama's meeting, which included Secretary of State John Kerry, Defence Secretary Chuck Hagel and others.

Hagel declined Friday to discuss specific force movements while saying that Obama had asked the Pentagon to prepare military options for Syria. U.S. defence officials told The Associated Press that the Navy had sent a fourth warship armed with ballistic missiles into the eastern Mediterranean Sea but without immediate orders for any missile launch into Syria.

Navy ships are capable of a variety of military actions, including launching Tomahawk cruise missiles as they did against Libya in 2011 as part of an international action that led to the overthrow of the Libyan government.

"The Defence Department has a responsibility to provide the president with options for contingencies, and that requires positioning our forces, positioning our assets, to be able to carry out different options — whatever options the president might choose," Hagel told reporters travelling with him to Asia.

Hagel said the U.S. is co-ordinating with the international community to determine "what exactly did happen" near Damascus. According to reports, hundreds were killed in a chemical attack in a suburb. It would be the most heinous use of chemical weapons since Iraqi leader Saddam Hussein gassed thousands of Kurds in the town of Halabja in 1988.

'Appears to be' chemical weapons

Hagel left little doubt that he thinks chemical weapons were used in Syria. "It appears to be what happened — use of chemical weapons," he said.

The United Nations disarmament chief, Angela Kane, arrived in Damascus on Saturday to press the Syrian government to allow UN experts to investigate the alleged chemical attacks.

Obama remained cautious about getting involved in a war that has killed more than 100,000 people and now includes Hezbollah and al-Qaeda. He made no mention of the "red line" of chemical weapons use that he marked out for Assad a year ago, which U.S. intelligence says has been breached at least on a small scale several times since.

"If the U.S. goes in and attacks another country without a UN mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it — do we have the coalition to make it work?" Obama said in a television interview broadcast Friday. "Those are considerations that we have to take into account."

Obama said in the interview with CNN's New Day that the episode is a "big event of grave concern" that requires American attention. He said any large-scale chemical weapons usage would affect "core national interests" of the U.S. and its allies. But nothing he said signalled a shift toward U.S. action.

'There may be another attack coming'

U.S. defence officials spoke on condition of anonymity because they weren't authorized to discuss ship movements publicly. But if the U.S. wants to send a message to Assad, the most likely military action would be a Tomahawk missile strike, launched from a ship in the Mediterranean.

For a year now, Obama has threatened to punish Assad's regime if it resorted to its chemical weapons arsenal, among the world's vastest, saying use or even deployment of such weapons of mass destruction constituted a "red line" for him. A U.S. intelligence assessment concluded in June that chemical weapons have been used in Syria's civil war, but Washington has taken no military action in response.

U.S. officials have instead focused on trying to organize a peace conference between the government and opposition. Obama has authorized weapons deliveries to rebel groups, but none are believed to have been sent so far.

In his first comments on Syria since the alleged chemical attack, Obama said the U.S. is still trying to find out what happened. Hagel said Friday that a determination on the chemical attack should be made swiftly because "there may be another attack coming," although he added that "we don't know" whether that will happen.

After rebels similarly reported chemical attacks in February, U.S. confirmation took more than four months. In this instance, a UN chemical weapons team is already on the ground in Syria.