AMMAN, Jordan — A pick-up truck rigged with explosives blew up near a Jordanian army post on the sealed border with war-ravaged Syria on Tuesday, killing six members of Jordan’s security forces and wounding 14, the military and a witness said.

It was the deadliest attack along the tense border in recent memory and raised new concerns about the pro-Western kingdom’s vulnerability to spillover from long-running conflicts next door.

There was no immediate claim of responsibility for the assault, the third against Jordanian security installations in the seven months. The previous two attacks had targeted security compounds inside Jordan.

The military said Tuesday’s explosion went off at 5:30 a.m. near an encampment for tens of thousands of Syrian refugees who are stranded in a remote area on the border and await entry into Jordan.

The attack targeted a military post serving Syrian refugees in an area known as Ruqban, the amy said.

A Ruqban resident said that sometime after 5 a.m., he saw a pickup truck speeding toward a Jordanian border gate and crashing through it. Seconds later, a blast went off, followed by the sound of shooting, said the resident who spoke to The Associated Press over the phone from the Ruqban area. He spoke on condition of anonymity for fear of repercussions from the authorities.

Cell phone photos from the camp show a cloud of gray smoke rising in the distance, with tents in the foreground.

Ruqban is the larger of two tent encampments that expanded rapidly in recent months as more Syrians fleeing fighting at home try to reach Jordan. The camps have also attracted smugglers, war profiteers and members of various armed groups fighting in Syria’s civil war.

The Jordanian military said those killed in Tuesday’s attack included four border troops, a member of the civil defense and a public security officer. The statement said 14 were wounded, including nine public security officers. It described the bombing as a “cowardly terrorist attack.”

Ruqban and the smaller Hadalat camp house about 64,000 Syrians, according to estimates by international aid agencies. Both camps are located near an earthen mound, or berm, that runs along the border. Ruqban is just a few miles from the point where Syria, Jordan and Iraq meet.