Massive security measures are in place for the June 28-29 summit and Turkish authorities say they have received no serious threat of a terrorist attack against the meeting which will be attended by US President George W. Bush and several other Western leaders.

"We have no intelligence of a serious threat, but we are nonetheless carrying out intelligence work and planned operations against underground groups. There is nothing to worry about," Istanbul governor Muammer Guler said in a television interview.

A direct attack against the gathering would be suicide for outlawed groups active in Istanbul as such a move would trigger a massive crackdown to annihilate the militants, according to Veli Fatih Guven, a terrorism expert from the Eurasia Strategic Studies Institute, an Ankara-based think-tank.

"But militants could carry out minor attacks aimed at intimidating civilians," he warned.

Istanbul, a city of more than 10 million people straddling the Bosphorus Strait, has been on edge since November last year when two twin suicide bombings -- only five days apart -- against two synagogues, the British consulate and a British bank claimed 63 lives and left hundreds more injured.

The carnage, the worst bomb attacks in the country's history, was blamed on local militants with links to Osama Bin laden's Al-Qaeda network.

In recent weeks, there have been a number of small bomb attacks in Istanbul and other Turkish cities, similar to those carried out in the past by left-wing militants.

In the run-up to the NATO summit, Turkish security forces have detained dozens in security sweeps across the country against several outlawed groups.

In May, a court in the northwestern city of Bursa charged nine men -- suspected members of the northern Iraq-based radical Islamist group Ansar al-Islam, which Washington says has links with Al-Qaeda -- in connection with plans to bomb the Istanbul summit.

Last week, four other suspected Ansar al-Islam members were detained in Istanbul with bomb-making equipment.

"Al-Qaeda and other extreme Islamist organizations have a grudge against the United States over allegations of abuse in Iraqi prisons and the escalating violence in the Middle East, but Turkey has good intelligence to stop them from acting out any plan," Guven said.

Besides Islamists, Turkey has also moved against Marxist militants behind a three-decade campaign of violence in the country.

Dozens of suspected members of the Revolutionary People's Liberation Party Front (DHKP-C) were arrested in coordinated raids earlier this year in Turkey and four other European countries.

Also targeted by police are militants from the Kongra-gel rebel group, the successor of the outlawed PKK which led a 15-year armed campaign for self-rule in Turkey's mainly Kurdish southeast until 1999 when it declared a ceasefire.

Kongra-gel announced last month that it was ending the truce on June 1.

Since then, police in the southern city of Adana have detained 12 suspected Kurdish rebels with plans to carry out bomb attacks.

"Both organizations are included in the European Union's list of terrorist organisation. It would be foolish for them to attack Western leaders," Guven said.

Nonetheless, Turkey is leaving nothing up to chance.

More than 23,000 police backed by military forces, manning warships and AWACS aircraft will be on duty for the summit which will take place behind steel and concrete barriers erected around a 10-kilometre (six mile) perimetre.

Air traffic over the venue will be banned and the Bosphorus Strait will be closed to oil tankers and ships with dangerous cargo.

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