Syrian rebels have held meetings with senior US government officials in Washington as pressure mounts on America to authorise a shipment of heavy weapons, including surface-to-air missiles to combat the Assad regime, The Daily Telegraph has learnt.

A senior representative of the Free Syrian Army met Robert Ford, the US ambassador to Syria, and Frederick Hoff, special coordinator for the Middle East, in the past week at the US State Department, sources have confirmed.

The rebel emissaries, armed with an iPad showing detailed plans on Google Earth identifying rebel positions and regime targets, have also met senior members of the National Security Council, which advises President Barack Obama on national security policy.

The rebels have compiled a “targeted list” of heavy weaponry, including anti-tank missiles and heavy machineguns that they plan to present to US government officials in the coming weeks.

The consultations come before next week’s G20 meeting in Los Cabos, Mexico, where British and US officials are expected to make a last-ditch attempt to get Vladimir Putin, the Russian president, to intervene in the Syria crisis.

Privately, Western diplomats admit they now harbour scant hopes of forcing a change of heart on Russia, which has steadfastly refused to bow to US and British pressure to do more to arrest Syria’s slide into sectarian civil war.

While there remains little appetite for direct Western military intervention, advanced contingency plans are already in place to supply arms to the rebels, including sophisticated anti-tank weapons and surface to-air-missiles.

The move towards what was described as a “Libya lite” intervention in Syria is expected to gather force following the expected failure of the Annan peace plan and the meeting of the Syria Contact Group scheduled for June 30 in Geneva.

Senior Middle Eastern diplomatic sources said that Libyan-supplied weapons, paid for by Saudi Arabia and Qatari government funds and private donations, had already been stockpiled in expected of the “inevitable” intervention needed to end the Assad regime.

“The intervention will happen. It is not a question of ‘if’, but ‘when’. The Libyans are willing to provide the anti-tank weapons, others are prepared to pay for it,” the source said

He added, however, that Turkey would “not open the floodgates” of acting as a conduit for the arms without Nato and US-backing that would guarantee them support in the event of a Syrian backlash, possibly mobilising Syrian Kurdish groups against Turkey.

Middle Eastern diplomatic sources said that the Obama administration was fully aware of the preparations being made to arm Syrian opposition groups.

The US has also agreed to be part of a group of countries that coordinates assistance to the rebels, the sources said, but was still deliberating over the time frame for escalation.

The Obama administration, which campaigned on a promise to end the wars in Iraq and Afghanistan, has been reluctant to approve military intervention in Syria as the president seeks a second term from a war-weary electorate.

However, proponents of arming the rebels are now arguing forcefully that US inaction leaves Mr Obama vulnerable to accusations from the Republican camp that he is “leading from behind” at the cost of thousands of innocent Syrian lives – a charge that would stick if there was another massacre.

Those in Washington who are lobbying on behalf of the rebel Free Syrian Army are aware of the limited political impetus for intervention in an election year, and that any deal would most likely need to be struck before influential congressmen return to their districts for summer recess in July.

Reports that heavy anti-tank weapons had been smuggled into Syria this week were denied by FSA sources who said that the rebels were still armed only with RPG-7 rocket-propelled grenades. However, it is understood that the contacts between rebels groups and senior US government officials have now reached the “getting to know you stage” as the administration faces the growing likelihood that it will have to sanction some kind of indirect intervention.

The US defence establishment is concerned that sophisticated weapons could fall into the hands of Islamist militants, or accelerate the cycle of sectarian revenge killings, rather than bring about the swift demise of the Assad regime.

The FSA has long been seen as the name given to a collection of disparate militias.

The movement has established a better command and control structure on the ground in recent months, setting up opposition military councils in 10 Syrian cities and towns, including in the capital.