The tribesmen seized the Turaibil crossing, known as the Karamah crossing by Jordan, late on Sunday.
It was not clear if the seizure of the only legal crossing point between Iraq and Jordan, was part of the broader advance by Sunni militants led by the Islamic State in Iraq and the Levant (ISIL).
The withdrawal of the Iraqi army from the Turaibil/Karamah border crossing left it with no presence along the entire western frontier, which includes some of the Middle East's most important trade routes.
Despite the instability, the crossing remains effectively open. Witnesses at the border on Wednesday said they were able to enter Jordan easily, and that Sunni tribesmen, not ISIL, were in control of various cities in Iraq.
“This thing they call ISIL is laughable, there is no such thing as ISIL. All the militants are tribesmen, in Baiji, in Fallujah, in Mosul, they're all armed tribesmen. ISIL are propaganda created by Maliki,” said truck driver Ismail Kuwailim after crossing into Jordan.
Jordanian army sources said their troops had been on a state of alert along the 180 km (112 mile) border with Iraq for several days to ward off “any potential or perceived security threats.”
Witnesses saw dozens of armored vehicles and scores of tanks on the highway heading to the Iraqi border crossing in what a Jordanian official said were reinforcements “that were sent in the last 24 hours in view of the latest developments.”