Jordan-Syria border (archive)
Jordan-Syria border (archive) Reuters

Jordan on Tuesday evening declared the desert border regions with Syria and Iraq "military zones" barred to civilians, after a suicide bomber killed six Jordanian soldiers near the Syrian frontier earlier in the day.

King Abdullah II had earlier vowed to hit back with an "iron fist" after meeting top civilian and military officials to discuss the attack in an area where thousands of Syrian refugees are stranded.

There was no immediate claim of responsibility but Jordan is part of the American-led coalition fighting the Islamic State (ISIS) group in Syria and Iraq, and has been targeted by ISIS jihadists before.

The Jordanian army said the dawn bombing killed four border guards, one member of the security services and one member of the civil defense. Fourteen soldiers were also wounded.

It said the suicide bomber set off from a makeshift Syrian refugee camp in no man's land near the Rukban border crossing in Jordan's remote north.

The driver entered Jordanian territory through an opening used for humanitarian aid deliveries and blew himself up as he reached a military post.

King Abdullah condemned the attack and said Jordan's armed forces would strike back.

"Jordan will respond with an iron fist against anyone who tries to tamper with its security and borders," he said.

"Such criminal acts will only increase our determination to confront terrorism and terror gangs that target army personnel who protect the security of the country and its borders."

Soon after, the army issued a statement declaring Jordan's desert regions that stretch northeast to Syria and east to Iraq "closed military zones".

"We will deal firmly with any vehicle of individual that moves in the area without (prior authorization) because they will be considered enemy targets," it warned.

The army did not explicitly say if the border with Syria would be closed.

But government spokesman Mohamed Momani told AFP the measure would not affect "humanitarian cases" -- a reference to refugees fleeing Syria's five-year war.

Tuesday's bombing comes two weeks after a gunman killed five Jordanian intelligence officers in a Palestinian refugee camp north of the capital.

A suspect was later arrested but details of the attack have been kept under a gag order while the investigation continues.  

Jordan has in the past carried out air strikes targeting ISIS jihadists and hosts coalition troops on its territory.

Maaz al-Kassasbeh, a Jordanian fighter pilot, was captured by the jihadists when his plane went down in Syria in December 2014 and he was later burned alive in a cage.

In March, Jordan announced it had foiled an ISIS plot to carry out attacks in the kingdom in an operation that led to the deaths of seven jihadists.

The U.S. embassy in Amman denounced Tuesday's bombing and pledged "unwavering support" for the armed forces of its key ally.

"We join the Jordanian people in their resilience and determination in the face of this cowardly terrorist act. The United States stands together with Jordan," it said.

AFP contributed to this report.