Six Jordanian security personnel have been killed in a suicide truck bomb attack near a makeshift refugee camp on the border with Syria, officials say.
The attack took place at about 05:30 (02:30 GMT) in the remote Rukban area.
Hours later, Jordan declared its northern and north-eastern borders would become closed military zones.
It is not yet clear who carried out the attack, but the jihadist group Islamic State (IS) has threatened in the past to "break down" Jordan's borders.
The kingdom is part of the US-led coalition against IS and has carried out air strikes on militants in Syria.
Tuesday's attack, the first of its kind since the conflict in Syria began in 2011, saw an explosives-laden truck driven at high speed over the border from Syria and blown up beside a Jordanian military post.
The blast left four border guards and two personnel from the Civil Defence and Public Security Department dead, a military statement said.
King Abdullah II said Jordan would "respond with an iron fist" against any group that sought to harm its borders or security.
Jordanian military chief Gen Mashaal Mohammed Zaben subsequently issued an order declaring the northern and north-eastern borders as closed military zones, a military source told the official Petra news agency.
The source said that any movement of vehicles or individuals in the military zones "without pre-coordination" would be "firmly dealt with as enemy targets".
Earlier this month, Jordanian intelligence service officers and two other employees were killed in a Palestinian refugee camp near the capital, Amman, in what the government said was a terrorist attack.
The Rukban camp is located beside an earthen berm in the desert, about 8km (5 miles) west of the point at which the Iraq, Syria, and Jordan borders meet.
The rocky area is devoid of shade, water or vegetation and is far from any towns.
Aid workers say more than 50,000 refugees are stranded at the camp, living in deplorable conditions while awaiting entry to Jordan.
UN satellite analysts mapped more than 6,400 probable shelters in the area in late April.
The Jordanian authorities currently only allow in about 50 to 100 refugees each day, citing security concerns.
Many of the refugees are believed to have fled areas controlled by IS in eastern Homs province and neighbouring Raqqa, where forces loyal to President Bashar al-Assad have made gains in the past year with the support of Russian air strikes.
Thousands of other refugees are stranded at another tented camp on the border at Hadalat, about 90km west of Rukban, where the UN mapped 1,900 shelters.
The UN acknowledged in December that Jordan had legitimate security concerns, but called on the country to allow all the refugees at the two camps to enter.
Jordan is hosting 655,000 of the 4.84 million Syrians registered as refugees with the UN.
The government says more than one million other Syrians are living there, including those who arrived before the uprising against Mr Assad began.