The US has left about 700 combat-equipped troops in Jordan after a training exercise, US President Barack Obama has said, after previously deciding to leave Patriot missiles and warplanes there.
Obama said on Friday that the deployment was done at the request of the Jordanian government, which fears a spillover of the war in neighbouring Syria into its territory.
An estimated 500,000 Syrian refugees have fled to Jordan to escape the bloodshed.
Citing officials, the AFP news agency said the US had expanded its presence in Jordan to 1,000 troops.
Speaking on condition of anonymity, a US defence official told the AFP news agency that "the total comes to about 1,000 [troops]," up from about 250 personnel that have been in place for months,
In a letter to House of Representatives Speaker John Boehner, Obama said about 700 of the troops deployed to Jordan as part of a training exercise that ended on Thursday would stay until the security situation improved.
"This detachment that participated in the exercise and remained in Jordan includes Patriot missile systems, fighter aircraft, and related support, command, control, and communications personnel and systems," Obama said.
'Friends of Syria' meeting
The developments come as 11 nations from the group 'Friends of Syria' meet in Qatar to discuss ways of providing military aid to Syrian rebels.
Washington is concerned about a possible spillover of violence from Syria into Jordan, its key ally and one of only two Arab states to have signed a peace treaty with Israel.
Obama has taken a go-slow approach on Syria but his administration has pledged to provide the rebels some light arms but not heavy-duty weaponry that some Republican legislators would prefer.
When the Pentagon first said last week it would leave Patriot missiles and F-16 fighters in Jordan, Russia, an ally of Damascus, said any attempt to use them to impose a "no-fly" zone over Syria would be illegal.
Obama has not ruled out participating in a no-fly zone that would prohibit the Syrian military from flying its aircraft, but he was skeptical about such a move in a television interview earlier this week.