Nato has begun to deploy Patriot missiles to Turkey to help Turkish troops repel attacks by missiles or aircraft from neighbouring Syria.
The US European Command said its troops and equipment had started arriving in southern Turkey, and more would arrive in the coming days.
Germany and the Netherlands are preparing to ship their Patriot batteries early next week.
The six battery units are scheduled to be operational by the end of January.
Nato approved the deployment of the surface-to-air missiles early last month, after a request from Turkey, amid "grave concerns" that Syria could use chemical weapons.
Syria has said it would never use such weapons against its own people.
But new launches of "Scud-type missiles" against rebel fighters were being detected in Syria, Nato said in mid-December.
Nato Secretary General Anders Fogh Rasmussen described it as "an act of a desperate regime approaching collapse" and said it emphasised "the need for effective defence and protection of our ally Turkey".
The US, Germany and the Netherlands have agreed to deploy two batteries of Patriot missiles each to be placed under the command of Nato along the Turkish-Syria border.
US personnel and equipment had begun arriving at Turkey's southern Incirlik Air Base and a further 400 personnel and equipment would be airlifted there in the coming days, the US command in Europe, Eucom, said. More equipment would reach Turkey by sea later in January, Eucom was quoted by the Associated Press as saying.
The Dutch Patriot batteries will depart for Turkey on Monday and are expected to arrive by 22 January along with nearly 300 troops, the country's daily De Telegraaf newspaper reports. Germany is expected to follow a similar schedule.
"The forces will augment Turkey's air defence capabilities and contribute to the de-escalation of the crisis along the Alliance's border," Eucom said in a statement.
"The deployment will be defensive only and will not support a no-fly zone or any offensive operation," it added.
The Syrian Observatory for Human Rights says more than 44,000 people have been killed since the uprising began nearly two years ago, including nearly 31,000 civilians
The UN believes up to four million people inside Syria are soon going to need humanitarian aid, up from 2.5 million. Another 500,000 Syrians have also fled to neighbouring countries.