* Traffic halted and unusual activity on Iraqi side, says Jordan minister
* Officials say gunmen seize control of sole land crossing with Iraq
* Jordan sees no immediate threat to loss of border to Sunni militants (Recasts with officials privately confirming border closed and army beefing up border area)
By Suleiman Al-Khalidi
AMMAN, June 22 (Reuters) - Jordan beefed up its border defences with Iraq on Sunday after Sunni gunmen seized territory close to its border in Anbar province and appeared to have also taken control of the only land crossing with its large eastern neighbour, officials and witnesses said.
Two officials said the border crossing almost 575 km (357 miles) from the Iraqi capital and nearly 320 km (199 miles) from Amman was effectively closed after Sunni gunmen took control of the crossing.
A Jordanian minister earlier told Reuters traffic had halted and there were signs of chaos at the crossing that serves as a major artery for passenger and trade flows between the two countries.
"The last traffic was around 7:30 pm (1630 GMT) and border officials are saying the situation is not normal on the other side of the border," Minister of State for Media and Communication Mohammad al-Momani said.
Earlier, Iraqi Sunni gunmen had seized control of the town of Rutba, just 90 miles (145 km) east of the border with Jordan.
An army source confirmed that army units had been put in a state of alert in recent days along the 181-km (112-mile) border with Iraq, redeploying in some areas as part of steps to ward off "any potential or perceived security threats".
Truck drivers who arrived in Jordan before traffic halted after crossing the border said Sunni tribal militants were now running and manning checkpoints along large stretches of the Baghad-Amman highway that runs through the crossing.
A security source who requested anonymity said the border crossing on the Iraqi side had fallen earlier in the day to local Sunni tribal gunmen who permitted customs officials to continue to run it administratively until later on Sunday.
U.S. ally Jordan was expected to formally close the border to traffic as soon as the Iraqi government formally announces its loss of control over the crucial trade and passenger crossing, he added.
On Sunday, militants overran a second frontier post on the Syrian border, extending two weeks of swift territorial gains as the Islamic State of Iraq and the Levant (ISIL) pursues the goal of its own caliphate straddling both countries.
ISIL thrust east from a newly captured Iraqi-Syrian border post on Sunday, taking three towns in Iraq's western Anbar province after seizing the frontier crossing near the town of Qaim on Saturday, witnesses and security sources said.
They seized a second, al-Waleed, on Sunday.
The gains have helped ISIL secure supply lines to Syria, where it has exploited the chaos of the uprising against President Bashar al-Assad to seize territory.
The loss of the Iraqi border crossing with Jordan was not seen as an immediate security threat to the kingdom although some were unnerved by the prospect of al Qaeda-affiliated groups along the border with Iraq, another official said.
It was difficult to see security-conscious Jordan, which has almost cut off any flow of militants across its heavily sealed northern border with Syria, allowing itself to become a launching pad or supply route for Islamist jihadists into Iraq, he added.
Jordan, grappling with the mounting impact of the grinding conflict in neighbouring Syria, is one of the closest U.S. allies in the Middle East. (Editing by Eric Walsh)