* Jordan's king says army prepared if Syria crisis worsens
* Army boosts presence on Syria border as U.S. deploys Patriots (Adds quotes, security official's comments)
By Suleiman Al-Khalidi
AMMAN, June 16 (Reuters) - King Abdullah said on Sunday Jordan's armed forces were ready to protect the country against any threat from the escalating civil war in neighbouring Syria.
He was speaking as Jordanian and U.S. forces conducted military exercises less than 75 miles (120 km) from the Syrian border, with the participation of 17 other countries.
Diplomats say the exercises, in their second week, aim to send a strong message to Syrian President Bashar al-Assad, who warned the kingdom last April not to use its territory as a launching pad against his forces.
A Pentagon spokesman said on Saturday Defense Secretary Chuck Hagel has approved a Jordanian request for U.S. F-16s and Patriot missiles to remain in the kingdom after the end of the military manoeuvres.
"If the world does not help as it should, and if the matter becomes a danger to our country, we are able at any moment to take the measures to protect the country and the interest of our people," King Abdullah told military cadets at a graduation ceremony in southern Jordan.
Jordan's military brass say the U.S.-led military exercises, in which more than 3,000 Jordanian troops are taking part alongside 4,500 American troops, are crucial to its military preparations in the coming months.
The king saluted members of Jordan's armed forces who helped the humanitarian relief operation to bring across the border hundreds of thousands of Syrian refugees fleeing bombardment in their towns and villages.
Jordan has taken in more than 500,000 Syrians out of a total 1.5 million who have fled the conflict in an exodus that has accelerated since the start of the year, United Nations officials say. They expect the number to double by the end of the year.
Jordan is one of a number of Arab countries that has lent support to the Syrian opposition and has channelled some arms to moderate rebel groups battling Assad's forces in southern Syria.
Like Syria's other neighbours, Jordan is increasingly nervous that the fighting will ignite a regional conflict.
The United States, which has long called for Assad to step down, pledged military support to Syrian rebels this week, citing what it said was the Syrian military's use of chemical weapons - an allegation Damascus has denied.
The decision to put Patriot batteries - an air and missile defence system - in Jordan has angered Russia, Assad's main international ally, which accuses the West of fanning the conflict in Syria.
Russia bristled at the possibility they could be used to enforce a no-fly zone inside Syria, a step Washington said was far harder and costlier to set up than it was in Libya, saying the United States had no national interest in pursuing that option.
A Jordanian security official told Reuters the kingdom has reinforced its army presence along its tightly sealed 370-km border with Syria as part of the deployment of the Patriot batteries and U.S. fighter jets.
"There are new military plans that will enhance our defensive capabilities to face any eventuality from Syria," said the source, without elaborating. (Editing by Janet Lawrence)