Mortars, rocket-propelled grenades and anti-tank missiles will be sent through friendly Middle Eastern countries already supplying the rebels, according to well-placed diplomatic sources.

The US has bought weapons from the stockpiles of Libya’s former dictator Muammar Gaddafi. They include SA-7 missiles, which can be used to shoot down aircraft.

The rebels are gaining ground after 20 months of civil war in which an estimated 40,000 Syrians have died. They have entered the suburbs of Damascus and have surrounded its airport. US State Department officials are in regular contact with rebel field commanders, talking to them on Skype for hours every day. The commanders have repeatedly pressed for more weapons.

President Barack Obama authorised clandestine CIA support earlier this year and both the US and Britain have had special forces and intelligence officers on the ground for some time.

They have helped with logistics and communications, but until now have refused to arm the Free Syrian Army, offering only “non- lethal assistance” such as humanitarian aid.

The US decision to supply weapons follows reports of movements at Syria’s chemical weapons sites. The White House is increasingly keen to hasten the end of the regime and ensure its influence with any post-Assad government.

Concerns that US-supplied weapons could fall into the hands of Islamists linked to al-Qa’ida have been partly eased by the formation of a National Coalition for Syrian Revolutionary and Opposition Forces, which will co-ordinate up to 300 rebel groups.

The move towards greater unity on the armed front comes as the US and others try to strengthen the opposition’s leadership while sidelining extremist factions that have become a vital part of the rebels’ ground forces.

The US will send in more advisers to help with tactics and manage weapons supplies. British advisers are also expected to be sent. The US and Britain are already training Jordanian and Turkish advisers to support the rebels.

The US is also intensifying diplomatic efforts to resolve the conflict. Secretary of State Hillary Clinton is due to attend a Friends of Syria meeting in Morocco on Wednesday, when the US is expected to recognise the opposition coalition as the legitimate representative of the Syrian people.

Meanwhile, rebel commanders from around Syria have joined forces under a united command they hope will increase co-ordination between diverse fighting groups and streamline the pathway for arms essential to their struggle against the Assad regime.

About 500 delegates elected the 30-member Supreme Military Council and a chief of staff at the weekend, and planned to meet representatives from the opposition’s newly reorganised political leadership, participants said.

It remains unclear how the new military command will relate to the opposition coalition and whether foreign powers will back it. But two of Syria’s most extreme rebel groups were not included: Jabhat al-Nusra, which has claimed deadly suicide bombings and is believed to be linked to al-Qa’ida, and Ahrar al-Sham, an Islamic fundamentalist brigade that is home to many foreign jihadis.

However, many of the participating groups have strong Islamist agendas, including the Tawheed Brigade, whose ideology is similar to that of the Muslim Brotherhood, and the Falcons of Damascus, an ultra-conservative Islamist group.

Additional reporting: AP