Syrian rebels have held meetings with senior U.S. government officials in Washington as pressure mounts on the U.S. to authorize a shipment of heavy weapons, including surface-to-air missiles to combat the Assad regime, the Daily Telegraph reports.

A senior Free Syrian Army representative met in the past week at the U.S. State Department with the U.S. ambassador to Syria, Robert Ford and Frederick Hoff, special coordinator for the Middle East, 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 with senior members of the National Security Council, which advises President Obama on national security policy.

FSA representatives in Washington have compiled a "targeted list" of heavy weaponry, including anti-tank missiles and heavy machine guns that they plan to present to US government officials in the coming two weeks.

The consultations come ahead of next week's G20 meeting in Los Cabos, Mexico where British and U.S. officials are expected to make a last-ditch attempt to get the Russian President Vladimir Putin to intervene in the Syria crisis.

Privately, western diplomats admit they now harbor scant hopes of forcing a change of heart on Russia, which has steadfastly refused to bow to U.S. 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, the Daily Telegraph has learned that advanced contingency plans are already in place to supply heavy weapons 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 anticipated 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 anticipation 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 U.S.-backing that would guarantee them support in the event of a Syrian backlash, possibly mobilizing 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 U.S. 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.