Leon Panetta, the US Defence Secretary, has signed an order to send two Patriot missile batteries to Turkey, with 400 American personnel to operate them, in a move by Nato to bolster Turkey's defences against the threat of Syrian missiles.

The order was signed shortly before Mr Panetta arrived on an unannounced visit to Turkey to meet American troops stationed at the Incirlik Air Base. His spokesman said: "The purpose of this deployment is to signal very strongly the United States, working closely with our Nato allies, is going to support the defence of Turkey, especially with potential threats emanating from Syria."

Nato member Turkey has repeatedly scrambled jets along the countries' joint frontier and responded in kind when shells from the Syrian conflict came down inside its borders, fanning fears the civil war could spread to destabilise the region.

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The US move follows similar steps by Germany and Holland and came as David Cameron said Britain is ready to step up its support for the Syrian opposition and warned "nothing is off the table" in the effort to speed the transition of power.

The Prime Minister was speaking at the end of a European Council summit in Brussels at which leaders agreed to look at all options for action to support a democratic future for Syria.