A car bomb exploded Tuesday near a Jordanian army post on the sealed border with war-ravaged Syria, killing six members of Jordan's security forces and wounding 14 in what the military called a "cowardly terrorist attack", reports The New York Times.

It was the deadliest attack along the tense border in recent memory and it raised new questions about the pro-Western kingdom's ability to block spillovers from long-running conflicts next door. There was no immediate claim of responsibility for the assault.

The military said the explosion went off at about 5:30 a.m. near a makeshift encampment for tens of thousands of Syrian refugees stranded in a remote desert area on the border, awaiting entry into Jordan.

The attack targeted a military post serving Syrian refugees in an area known as Ruqban — the larger one of two tent encampments that expanded rapidly in recent months as more Syrians fleeing fighting at home try to get into Jordan.

Cell phone photos taken from Ruqban showed a cloud of gray smoke rising in the distance, with tents in the foreground.

The two camps are located along 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.

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."

It was not immediately clear if the attack would disrupt daily deliveries of food, water and other essentials by international aid agencies to the refugees.

Ariane Rummery, a spokeswoman for the U.N. refugee agency in Geneva, said the attack took place several kilometers from Ruqban camp and that she was not aware of any injuries among the Syrian refugees.

"Clearly the attack underlines how challenging the relief operation is at the berm," she said. "We remain concerned about the level of security issues at the berm and for humanitarian agencies working there."

Several aid groups, including the World Food Program, said they were not able to reach distribution points near the berm Tuesday.

With crowds at the berm swelling rapidly in recent weeks, the situation for refugees in the two encampments has become increasingly dire.

Jordan only admits a limited number of refugees every day, citing the need for stringent security vetting. Jordanian officials have said they have evidence that activists of the extremist group Islamic State have infiltrated the two camps and are attempting to slip into Jordan, pretending to be refugees.

Islamic State controls large areas in neighboring Syria and Iraq. Jordan has fortified border defenses, including with U.S.-funded surveillance systems, to try to stop attackers and infiltrators.

Jordan has also widened a crackdown on IS sympathizers at home, jailing hundreds in the past two years for promoting the group's ideas on social media.

The kingdom is a member of the U.S.-led international military coalition against IS.