A draft resolution authorizing "necessary measures" in Syria will be presented to the U.N. Security Council within hours, British Prime Minister David Cameron announced Wednesday.
The move came as the U.S. and its allies lay the diplomatic groundwork for military strikes, which officials in Washington said could begin as early as Thursday night.
"Britain has drafted a resolution condemning the chemical weapons attack by President Bashar Assad and authorizing necessary measures to protect civilians," Cameron said on his official Twitter feed.
It sets the scene for a diplomatic showdown with Russia - a key suppliers of weapons to Assad's regime - and China, which has a veto on the Council and is opposed to military strikes.
"We've always said we want the UN Security Council to live up to its responsibilities on Syria. Today they have an opportunity to do that," Cameron added.
However, U.N. Secretary-General Ban Ki-moon said inspectors should be given time to determine whether Assad used poison gas against civilians.
"It is essential to establish the facts. The team needs time to do its job," he said in The Hague on Wednesday.
He spoke as the inspectors worked in rebel-held territories east of Damascus to collect more evidence of a suspected poison gas attack on Aug 21 that left hundreds dead.
However, the U.S. State Department said Tuesday it was already "crystal clear" that Assad's government was responsible. Vice President Joe Biden went even further, bluntly telling an American Legion audience in Houston: "Chemical weapons have been used."
Syrian President Bashar Assad has denied that his forces used chemical weapons.
In preparation for a likely three-day wave of military strikes, President Barack Obama and senior members of his administration have made at least 88 phone calls to allies and world leaders since disturbing pictures emerged of the Aug 21 deaths.
While the Arab League issued a statement Tuesday blaming Assad for the attack, it stopped short of endorsing military retaliation – leaving Obama with less support in the region than he had for the 2011 air strikes on Libya.
In anticipation of possible regional fallout, security forces in Israel moved an Iron Dome defense shield towards its northern border Wednesday, Israeli news site Ynet reported, and raised the alert level on its Arrow 2 missile battery.
Meanwhile, a terror group linked to al Qaeda pledged a "volcano of revenge" against Syria. A branch of the Islamic State in Iraq and the Levant said it would attack Syrian government security and military targets, according to a statement highlighted by the SITE monitoring service and reported by Reuters.
Top U.S. national security aides gathered to review the situation on Tuesday night in a meeting chaired by Obama's national security adviser Susan Rice, officials told Reuters.
Navy officials said four destroyers are lined up ready to strike: the USS Barry, the USS Mahan, the USS Ramage and the USS Gravely.
Tuesday, a fifth guided-missile destroyer, the USS Stout, also entered the Mediterranean, through the Straits of Gibraltar, but officials said it wouldn't take part in any cruise missile attack.
"The four destroyers now in place have more than enough cruise missiles," one official said.
Experts say UN inspectors will painstakingly scour the soil for evidence of nerve agents as they investigate last week's alleged chemical attack.
Craters where munitions exploded and shell fragments could provide clues as to who was behind the incident. The meticulous evidence-gathering process is likely to be filmed and exact locations where samples were found will be recorded using GPS technology.
A "chain of custody" will ensure the samples cannot be tampered with. Several witnesses will watch as evidence is placed in sealed containers. They will not be re-opened until they reach the laboratory. Samples are likely to be sent to several sites in the U.S., U.K., Finland, Netherlands and Switzerland.
Professor Alastair Hay, a U.K.-based chemical weapons expert who has conducted six investigations into alleged incidents, said that once the evidence has been gathered the testing process could take anywhere from a few days to several weeks. That raises the possibility that the U.S. and its allies could strike at Syrian targets before the UN has concluded its inquiry.
NBC News' Paul Goldman, Jim Miklaszewski, Courtney Kube and Charlene Gubash, and Reuters, contributed to this report.