Speculation is growing that the United States is preparing an alternative military plan for Syria amid doubts that its pledge last week to arm the rebels would have much effect on the balance of power in the country.

If the Americans do decide to intervene more directly in Syria, it now has the forces in place to do so. Last week, the US carried out a military exercise near the Syrian border together with the Jordanian army and an 8,000-strong international force.

American F-16 fighter jets and Patriot air-defence missiles are stationed indefinitely in Jordan and these could be used to establish a no-fly zone at short notice, along with the US forces already stationed in Turkey.

Syrian sites used for the storage and preparation of chemical weapons could also be attacked from the air, should President Barack Obama give the order. The chances of the US acting without a UN mandate, however, seem slim given Russian President Vladimir Putin’s steadfast opposition to intervention at the G8 summit this week.

The US announcement last week that it would supply the rebels with weapons followed its acknowledgement that the Assad regime has used chemical weapons and thereby crossed Mr Obama’s “red line”.

US officials said that only light weapons and ammunition will be transferred to the rebels and that the administration is only going to aid groups that have been “vetted” and who are not jihadists.

However, at the moment there is no process for vetting the various rebel groups. An estimated 300 different battalions are currently fighting in Syria, and even if the administration finds a way of differentiating between them, the groups frequently fight together and there is varying degrees of co-operation between the secular fighters and the Islamists. The Americans have no way of preventing arms being transferred between them.

The US has three allies bordering Syria — Israel, Jordan and Turkey. For diplomatic reasons, arms shipments through Israel are unlikely and, while Jordan could be a channel for weapons to rebels fighting near its border, the key battles are going on in the north of Syria, especially around the cities of Aleppo, Hama and Idlib. That leaves only the Turkish border, but since there is currently no agreement on a no-fly zone, the arms would have to pass via small smuggling routes that cannot handle large consignments.

Meanwhile, the US is still opposed to supplying the rebels with advanced anti-aircraft missiles. These could potentially cancel the advantage the Syrian regime’s current air-force advantage over the rebels.

In recent days, the administration has been criticised in Washington for doing “too little, too late” to help the Syrian rebels. So far, it has been prepared to take only limited steps, but the stationing of forces in Jordan indicates that it intends to be ready for any eventuality.