Six Jordanian border guards were killed by a suicide bomber who drove a car at a high speed across the border from Syria and rammed it into a military post on Tuesday, security officials said.
The explosives-laden vehicle blew up a few hundred metres from a camp for Syrian refugees in a remote, desolate area where the borders of Iraq, Syria and Jordan meet, a Jordanian army statement said.
The southeastern desert area is close to where Islamic State militants are known to operate, according to a security source who requested anonymity. The source said the attack appeared to be a well-planned military operation. No group has claimed responsibility.
The army said a number of other vehicles used in the attack at around 5:30 am (0230 GMT) were destroyed and that 14 other people were wounded. The suicide bomber drove out from behind a berm and dodged gunfire to reach the military post, it added.
It was the first such assault targeting Jordan from Syria since Syria's descent into conflict in 2011, and followed an attack on June 6 on a security office near the Jordanian capital Amman in which five people, including three Jordanian intelligence officers, were killed.
The incidents have jolted the Arab kingdom, which has been relatively unscathed by the instability that has swept the Arab world since 2011, including the expansion of Islamic State in Syria and Iraq.
In a rare move, the Jordanian army's chief of staff declared the northern and northeastern border strip with Syria a closed military zone, an order that went into effect immediately.
"Any vehicle and personnel movement within these areas that move without prior coordination will be treated as enemy targets and dealt with firmly and without leniency," the army statement said.
International relief workers said the Jordanian authorities had also suspended all humanitarian aid to the area and that this could put the lives of refugees at risk.
UN agencies responsible for the well-being of thousands of refugees did not comment on the drastic move which Western aid workers said penalized thousands of refugees, almost half of them women and children who have been stranded on the border strip for months.
Jordan's foreign minister Nasser Joudeh confirmed deliveries of aid to the border areas had been halted until a safer area was found.
He told state media the incident vindicated Amman's previous security warnings about the presence of hardline extremists within the camp.
"There is a large concentration of people along this border and a big infiltration of elements from Daesh terrorists who are present heavily," Joudeh said, using the Arabic derogatory term for Islamic State.
Jordan's King Abdullah said the perpetrators would not go unpunished and that his security forces would deal with "an iron fist" with any group that sought to harm the country's security or borders, a palace statement said.
Jordan is a staunch ally of the United States and is taking part in the US-led campaign against Islamic State in Syria, where the jihadist group still controls large areas of territory including much of the east.
Jordan has kept tight control of its frontier with Syria since the outbreak of the war in its neighbour.
Washington condemned the deadly attack as a "cowardly terrorist act" and said it would continue "unwavering support" for the Jordanian army, a statement from the US embassy said.
Since the Syria conflict began, Washington has spent tens of millions of dollars to help Amman set up an elaborate surveillance system known as the Border Security Programme to stem infiltration by militants from Syria and Iraq.
The Rakban crossing targeted on Tuesday is a military zone far from any inhabited area, and includes a three-kilometre (two-mile) stretch of berms built a decade ago to combat smuggling. The border is heavily guarded by patrols and drones.
US Patriot missiles are stationed in the kingdom, however, and the US army has hundreds of trainers in the country.
It is the only area where Jordan still receives Syrian refugees, some 50,000 of whom are stranded in Rakban refugee camp in a de facto no-man's land some 330 kilometres (200 miles) northeast of Amman.
REFUGEES STRAIN KINGDOM
The camp's population has grown from several thousand to over 50,000 people since last year as the fighting in Syria intensified, relief workers say.
Jordan has been a big beneficiary of foreign aid because of its efforts to help refugees, but has drawn criticism from Western allies and aid agencies over the humanitarian situation at Rakban where conditions have caused several deaths among refugeees lacking sufficient medical care, diplomats say.
Earlier waves of Syrian refugees had an easier time, with some walking just a few hundred metres to cross into Jordan. Jordan sealed those border crossings in 2013.
The United Nations refugee agency said late last year Jordan should accept the new wave of refugees -- their numbers have risen, aid officials say, since Russia started air strikes last September -- and move them to established camps closer to Amman.
Jordan, which has already accepted more than 600,000 UN-registered Syrian refugees, is resisting. It says IS militants may have infiltrated their ranks as most of them come from Islamic State-held areas in central and eastern Syria, and has allowed only a trickle of refugees, mostly women and children, in recent months.