'Eager Lion' begins weeks after first US intervention in neighbouring Syria and Iranian claims of a planned incursion through Jordan
Jordan and the United States kicked off annual military exercises known as "Eager Lion" on Sunday, with about 7,400 troops from more than 20 nations taking part, officials said.
US and Jordanian officials said the manoeuvres would include border security, cyber defence, and "command and control" exercises to bolster coordination in response to threats including terrorism.
"Joint efforts and coordination and the exchange of expertise ... are needed at the time when the region is facing the threat of terrorism," said Khalid al-Shara, a Jordanian brigadier general.
Bill Hickman, the US deputy commander for the US army in the region, said this year's "Eager Lion" exercises, the seventh since the first held in 2011, are "the largest and most complex to date".
The exercises come amid reports that Iran had offered to send troops to the southern areas of Syria to support the government of Bashar al-Assad in response to a reported buildup of US backed-forces near Jordan's border.
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The reports in the Tabnak news agency, which is affiliated to a former commander of the powerful Iranian Revolutionary Guards Corps, indicated Iran had growing concerns about a buildup.
"There are increased activities in southern Syria that indicate preparations for an attack through Jordan and Israel and [with the help of] armed groups," the report read.
Journalist Asaad Hanna tweeted earlier this month that US forces had been positioned and were standing by on the southern Syrian border with Jordanian special forces.
Another report in al-Hayat also indicated a joint force was preparing to enter southern Syria from Jordan, albeit ostensibly with the aim of combating the Islamic State (IS) group.
The exercises also come weeks after the US launched a cruise missile attack on a Homs airbase, the first direct intervention against Assad, in response to a chemical weapons attack on the Idlib town of Khan Sheikhun.
The highlight of this year's exercise, Hickman said, will be that "for the first time ever a global strike mission" will be conducted by "two US Air Force B-1B bomber aircraft," a long-range multi-mission bomber.
A statement by the Jordanian army said troops from Europe, Asia, Africa and the Arab Gulf region are taking part in the exercises, which run to 18 May, including from Britain, Japan, Kenya and Saudi Arabia.
About 6,000 troops from Jordan and the US took part in last year's exercises.
Jordan is a key recipient of US financial aid and a partner in the US-led coalition battling the Islamic State group in Syria and Iraq.
US forces have trained a small group of vetted Syrian rebels in Jordan, and American instructors have trained Iraqi and Palestinian security forces in Jordan as well over the past few years.
Two years ago, the United States announced its intention to increase overall US assistance to Jordan from $660m to $1bn annually for the 2015-2017 period.