U.S. Defense Secretary Chuck Hagel said U.S. forces were "prepared to exercise whatever option" President Barack Obama ordered.
British Foreign Secretary William Hague said London, Washington and their allies were militarily poised to deliver a harsh military message to Syrian President Bashar Assad after Wednesday's alleged nerve gas attack on a Damascus suburb that the Syrian opposition says killed more than 1,100 civilians.
"We cannot in the 21st century allow the idea that chemical weapons can be used with impunity, that people can be killed in this way and that there are no consequences for it," he said.
The Assad regime denies using chemical weapons.
Syrian ally Russia has accused the rebels of staging the attack, which the Syrian opposition says began shortly after 2 a.m. when Russian-type Grad rockets, similar to those the Palestinian Sunni Islamic group Hamas has fired into Israel, were launched.
Obama spoke with British Prime Minister David Cameron, French President Francois Hollande and German Chancellor Angela Merkel over the weekend. All the leaders agreed on the need for a "serious response" to the alleged attack, officials said.
Obama and Hollande expressed "grave concern" about the reported chemical-weapons use, the White House said Sunday.
It was not immediately clear whether France and Germany would participate in any military action against Syria.
Syria warned against any outside intervention, saying it would create "a ball of fire" that would "inflame the Middle East."
Russia cautioned against military action without U.N. approval, saying countries wouldn't want to repeat the 2003 U.S.-British-led coalition "mistakes" in the invasion of Iraq, whose justification was to remove weapons of mass destruction that were never found.
U.S. officials told The New York Times they would hold consultations at the United Nations, but made clear the world body was only one avenue for taking action against Syria.
Iran predicted "harsh consequences" if the United States intervened against the Iranian ally.
Israeli Prime Minister Binyamin Netanyahu said Jerusalem would strike back if Damascus turned its weapons on Israel, while President Shimon Peres called for an international effort to "take out" Syrian chemical weapons.
No final military decision was made, U.S. officials said Sunday night.
British newspapers The Daily Telegraph and the Financial Times said Western military action could take place as early as this week.
The Western allies did not plan a sustained military intervention, the Financial Times and The New York Times said.
A senior Obama administration official said in a statement Sunday Washington and the allies had "very little doubt" Assad's military forces used chemical weapons against civilians and said Syria's promise Sunday to give U.N. inspectors access to the sites was "too late to be credible ... because the evidence available has been significantly corrupted as a result of the regime's persistent shelling and other intentional actions" since the alleged gas attack.
Hollande told Obama "everything was consistent with naming the Damascus regime as the author" of the alleged chemical attack, a statement from the French government said Sunday.
The most likely military option involves long-range cruise-missile strikes from a U.S. destroyer and other military watercraft in the Mediterranean Sea because the Syrian air force is widely considered strong enough to shoot down enemy jets, officials told several news organizations.
A British nuclear-powered submarine is in the region and a number of British warships are traveling to the Mediterranean for exercises, the Telegraph said.
Targets discussed include missile or artillery batteries that launch chemical munitions, along with symbols of Assad's power, such as government headquarters and offices, officials told The New York Times.
Obama warned a year ago a Syrian use of chemical weapons would cross a "red line."
The White House conclusion in June Assad forces used chemical weapons on a small scale a number of times in the past year did not bring about a noticeable shift in U.S. engagement.
Obama, who came into office the United States was prosecuting wars in Iraq and Afghanistan is reported extremely reluctant to commit U.S. military forces, even in the form of missile strikes, to another Middle East conflict.
U.N. inspectors already in Syria looking into earlier claims of chemical-weapons use planned "to conduct on-site fact-finding activities" Monday in the Damascus suburb allegedly attacked, a U.N. statement said.