By Tim Cocks

MUNTHERIA, Iraq, Nov 20 (Reuters) - Whether it's al Qaeda militants entering through Syria or Shi'ite militia smuggling guns or bombs from Iran, Iraq's porous borders pose a major security headache.

U.S. and Iraqi forces say they are ratcheting up policing of Iraq's frontiers to stem the flow of militants and weapons, taking advantage of security gains across the country.

"It wasn't too long ago, they couldn't talk about problem areas on the border, because border security didn't exist," U.S. Brigadier-General Keith Walker, commander of U.S. teams embedded with Iraqi forces, told Reuters.

"Now, because security is better in other places, we're re-allocating resources to the borders," he said.

At Muntheria crossing point, along the rugged, mountainous border with Iran, a line of fuel trucks snakes into Iraq.

On the Iranian side, They drive past a mural adorned with the face of Ayatollah Ruhollah Khomeini -- Iran's leader during its eight year war against Saddam Hussein's Iraq in the 1980s.

They pass the border gate, and wait to be scanned by an X-ray machine installed in a white van.

"We're looking closely for weapons and explosives, also drugs," said Brigadier-General Khalid Salman Kumer, the Iraqi army officer in charge of security at the point of entry.

"We have three X-ray machines to scan all the trucks, with two agents at the screen. It takes about an hour."

At least 2,000 people a day cross here, mostly Iranian pilgrims on trips to visit holy sites of Shi'ite Islam in Iraq.

A sea of pilgrims, many of them women wrapped from head to toe in black shrouds, wait at the gate. Iraqi police search their luggage then others check their passports. Their details are checked against databases of criminal suspects.

"It's taking long, but we don't mind. I'm excited to visit the shrines of the Imams," said Ali Khusrawi, 74. "It's my first time for five years because Iraq was too dangerous before."

SECURITY WOES

Nestled on the edge of Iraq's volatile, ethnically and religiously mixed Diyala province, Muntheria is a microcosm of the many security woes the country faces. Sunni Islamist al Qaeda are active in the area -- a September bomb attack on Khanaqin, 10 km (6 miles) away, killed eight people.

Khanaqin was also the scene of a tense standoff between Kurdish forces and Iraqi security forces in August.

But most troubling for border security officials are Shi'ite militias they say are smuggling weapons and fighters from Iran.

U.S. officials have long accused Iran of arming, financing and training small Shi'ite militia units which attack U.S. troops and Iraqi forces. Tehran denies the charge, but U.S. officials say they often find Iranian weapons in seized caches.

"Frankly, I don't see any cooperation from the Iranian side stopping criminals," Kumer said. "They train them, back them, they give them weapons ... It's up to us to stop them."

Last week, border guards caught two entering, he said.

Iran says it does not back Iraqi militias and is being made a scapegoat for the failure of the U.S. military to secure Iraq.

John Mansell, an official at the U.S. Department of Homeland Security working on border security at Muntheria, said Iranian officials sometimes hold back traffic then open the gates after a big queue has built up. That creates confusion.

"What they do is try to facilitate moving people around during these confusing times. Enablers on this side are then moving people through to avoid the checks," he said.

But in recent months, a more rigorous system of checks and scanner systems installed at the border crossings have made smuggling arms much harder at official crossing points, Walker said. Militants were resorting to crossing in remote areas. (Editing by Dominic Evans)

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