INTERVIEW-Foreign militants entering Iraq down 50 pct-Petraeus

BAGHDAD, Feb 11 (Reuters) - The flow of foreign militants entering Iraq to fight for al Qaeda has fallen by half, General David Petraeus, the U.S. commander in Iraq, said on Monday.

Much of the fall in numbers was due to countries barring young men from flying to the Syrian cities of Damascus and Aleppo on one-way tickets, said Petraeus.

The U.S. government has long urged Syria to take steps to stop foreign fighters crossing its long land border with Iraq. Damascus says it has stepped up security on the frontier after the U.S. criticism.

"The flow of fighters is down, we think by about 50 percent," Petraeus told Reuters in an interview.

"It's a result not just of Syrian activity, although there has been some. It's the result of source countries making it tougher to fly as a military age male," he added.

Most of the fighters coming across the borders are believed to be affiliated with Sunni Islamist al Qaeda in Iraq. Al Qaeda's foot soldiers and suicide bombers attack U.S. forces, the Shi'ite-led government and groups they regard as un-Islamic.

Last month the U.S. military said captured al Qaeda documents showed 750 militants from 22 countries entered Iraq during the 12 months leading up to August 2007.

Petraeus did not say which countries were taking action at airports, but the documents said just under half the foreign fighters entering Iraq come from Saudi Arabia, followed by Libya, Yemen, Syria, Tunisia and Morocco.

It also said 90 percent of the al Qaeda suicide bombers were foreign. Most of al Qaeda in Iraq's leadership is foreign while the rank-and-file are Iraqi, the U.S. military says.

Petraeus said suicide bombers were still getting through, although he said it was difficult to assess numbers.

Some of those fighters enter Iraq through northern Nineveh province, which has become a key haven for al Qaeda after its militants were forced to flee military offensives in western Anbar province and Baghdad.

Petraeus said Nineveh was the only province in Iraq where attacks had not fallen in recent months.

"In fact they have actually gone up," Petraeus said without giving details.

The capital of Nineveh is Mosul, Iraq's third largest city and where Prime Minister Nuri al-Maliki has promised a final, "decisive" battle against al Qaeda.

But Petraeus cautioned that it would take months.

"This is not going to be a lightening campaign .. you don't clear a city of 1.6 million the way you clear a city of 200,000 or 400,000," he said.

The U.S. military had deployed an extra 1,500 to 2,000 troops to Nineveh in the past several months, Petraeus said. Iraq has also sent additional forces.

"Al Qaeda clearly has sustained very serious blows. I don't think there is any question about that," Petraeus said.

"But they remain lethal, we regard them still as public enemy number one in Iraq. They are the wolf closest to the sled." (Editing by Samia Nakhoul)