Six Jordanian border guards were killed by a car bomb in a remote area of the frontier with Syria on Tuesday during an attack launched from Syrian territory, security officials said.
The explosives-laden vehicle blew up a few hundred metres from a camp for Syrian refugees in a desolate eastern area of Jordan 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 asked Reuters for anonymity. The source said the attack appeared to be a well-planned military operation in which several suicide bombers were involved. No group has claimed responsibility.
The army said a number of other vehicles used in the attack were destroyed and that 14 other people were wounded in the attack at around 5:30am.
It was the first attack of its kind 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.
Jordan's King Abdullah vowed that the perpetrators will not go unpunished and said his security forces will 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 as a "cowardly terrorist act" the deadly attack saying it would continue "unwavering support" for the Jordanian army, a statement from the US embassy said.
Since the start of the Syria conflict, 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 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 200 miles northeast of Amman.
The population of the camp has since last year grown from several thousand to over 50,000 people 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, 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 Islamic State 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.