A task force of 300 US Marines has been deployed on the Syrian border after President Obama approved a move to arm rebels fighting to end the regime of Bashar al-Assad.
A Patriot anti-aircraft missile system was also moved to the area, north of Al-Mafraq in northern Jordan, as the West prepares to send in weaponry and armaments to the rebels.
The troop movements have taken place under cover of a military training exercise being held this week, but will remain in place for months, according to White House sources.
The troops and equipment are intended to increase stability in the region rather than to train rebel fighters or launch operations inside Syria, US officials said.
Western involvement in the conflict was being stepped up last night after Obama's decision, which followed confirmation that Assad's regime had used chemical weapons in defiance of Washington's "red line".
Russia condemned the move, with foreign minister Sergei Lavrov saying it could cause an escalation of the conflict.
David Cameron said analysts at Porton Down, Britain's chemical warfare testing centre, had concluded Assad's forces used the nerve agent sarin on two occasions: at Utaybah on 19 March, and at Sheikh Maqsood on 13 April.
Cameron highlighted the dangers of chemical weapons spreading, and being used more widely. He warned that extremists linked to the rebels were seeking to procure nerve agents.
"Elements affiliated with al-Qaida have attempted to acquire chemical weapons for probable use in Syria," said Cameron.
US military planners are gearing up for the most high-risk operation since the beginning of the Arab Spring.
The Pentagon is drawing up a list of weapons to supply to the rebels, including shoulder-launched anti-tank missiles, ammunition and command-and-control systems.
"American arms will not be a massive game-changer," said one British official, pointing out that the Saudis and Qataris had been arming the rebels for months. "The rebels can't be given high-tech weaponry. They won't have the training to utilise them."
US Defence Secretary Chuck Hagel is to approve stationing a dozen F-16 fighter jets and the Patriot missile battery in Jordan after the exercise, possibly along with several hundred more US troops on top of those already stationed in the country over the past two years.
UN Secretary General Ban Ki-moon criticised Obama's decision, saying that arming either side "would not be helpful". "Providing arms to either side would not address this current situation. There is no such military solution," he said.
Western leaders are holding emergency talks over how and when to arm the rebels and whether to impose a no-fly zone.
Obama spoke to Cameron along with President Hollande of France, German Chancellor Angela Merkel and Italian prime minister Enrico Lette for almost an hour, according to an aide to the French leader.
The leaders "discussed the situation in Syria and how G8 countries should all agree to work together on a political transition to end the conflict", Downing Street said in a statement.
Diplomatic activity will then shift to the G8 summit in Northern Ireland, where Russian President Vladimir Putin will face pressure from Cameron, Obama and Hollande to force the Assad regime to talks in Geneva.
Cameron is to meet Putin in Downing Street on Sunday, before the summit begins on Monday.
The White House said its aim was not to bring about outright victory for the rebels, but to force Assad into negotiations by making his victory impossible.
It hopes regime figures can play a role in a future Syrian administration, once they accept the end of the current regime.
Washington is sceptical about the benefits of imposing a no-fly zone, which officials said would be more complex than had been the case in Libya, given the sophisticated Russian-made air defences at Assad's disposal.
Moves to arm the rebels were likely to have come too late to save Aleppo from Assad's forces, who appeared poised to seize control of the city on Saturday 15 June.