In recent weeks, Syrian rockets and shells have landed on the Turkish side of the border, killing several people, a development that US Defense Secretary Leon Panetta has said raises concerns that the Syrian conflict could spill over into other countries in the region.

Meanwhile the war inside Syria intensifies as 11 people were killed late on Thursday by a blast at a station packed with Syrians queuing for fuel, which has become scarce in the 21-month insurgency against Assad, opposition activists said.

The U.S. troops, who began arriving at Incirlik air base in Turkey, will man two U.S. Patriot batteries out of a total of six batteries that have been promised by NATO allies (Netherlands, Germany) according to AFP.

“The purpose of this deployment is to signal very strongly that the United States, working closely with our NATO allies, is going to support the defense of Turkey, especially with potential threats emanating from Syria,” Pentagon Press Secretary George Little said today.

“Both the United States and Turkey share common concerns now about the violence in Syria and the threat that it poses to regional stability, he added

Germany and the Netherlands are also providing two Patriot batteries and up to 400 troops each.

NATO foreign ministers approved Turkey’s request for the Patriot deployment in early December with ministers saying the goal is to “defend the population and territory of Turkey and contribute to the de-escalation of the crisis along the alliance’s border.”

NATO Secretary General Anders Fogh Rasmussen described the situation along Turkey’s southeast border with Syria as one of “grave concern,” adding that NATO stands in full solidarity with Turkey.

The decision comes as the civil war in Syria intensifies and amid new concerns in Washington over the status of the Syrian government’s stocks of chemical and biological weapons.

The United Nations recently estimated that more than 60,000 people have been killed since the uprising began nearly two years ago.

The situation along NATO’s south-eastern border and the repeated violations of Turkey’s territory raise grave concern, NATO foreign ministers said in a statement.

Ministers declared their determination to deter threats to, and defend, Turkey. They welcomed the intention of Germany, the Netherlands and the United States to provide Patriot missile batteries, subject to their respective national procedures. Once deployed, the systems will be under the operational command of NATO’s Supreme Allied Commander in Europe (SACEUR).

“Turkey has asked for NATO’s support, and we stand with Turkey in the spirit of strong solidarity,” NATO Secretary General Anders Fogh Rasmussen said in a press conference.

“To the Turkish people we say: we are determined to defend you and your territory. To anyone who would want to attack Turkey we say don’t even think about it”, he said.

The Secretary General stressed that the deployment will be defensive only, will in no way support a no-fly zone or any offensive operation and will contribute to the de-escalation of the crisis along the Alliance’s border.

The agreement follows consultations in the framework of Article 4 of the North Atlantic Treaty held at Turkey’s request. The North Atlantic Council will regularly assess the developing situation and the implementation of deployment.

“NATO’s ultimate task is the protection and defence of our members. Today, we underline once again our steadfast commitment to the security of this Alliance and our full solidarity and resolve to protect our populations and territories,” Mr Rasmussen underlined.

Mr Rasmussen said that NATO Foreign ministers unanimously expressed grave concern over reports that the Syrian regime may be considering the use of chemical weapons and underlined that any such action would be completely unacceptable and a clear breach of international law.