United Nations weapons inspectors have carried out a surprise inspection at a helicopter base more than 30 kilometres (18 miles) north of Baghdad.

They suspect crop-spraying aircraft may have been converted to use biological or chemical weapons at the base at Khan Beni-Saad.

The BBC's Ben Brown says this was one of the most dramatic inspections so far.

The UN experts drove in at high speed through the gates of the base, and then blocked the entrance with one of their four-wheel drive land cruisers.

The Iraqis insist they have nothing to hide

No-one was allowed in or out while they carried out their mission.

The inspectors resumed their work on Wednesday after a four-year absence, operating under a new UN Security Council mandate that gives Iraq a "final opportunity" to shut down its chemical, biological and nuclear weapons programmes or face "serious consequences".

The Iraqis have co-operated with the new inspection regime, though under protest.

British accusations

Many helicopters were sitting on the runway at the base, apparently used for agricultural crop spraying.

But the recent British government dossier on Iraq says that before the Gulf War Iraqi scientists carried out studies on how to spray biological weapons from both helicopters and other aircraft.

The dossier also claims Iraq is probably now able to disperse by aerosol spray chemical and biological weapons over a large area.

This is the fourth day of weapons inspections. So far the Iraqis still appear to be co-operating.

Diplomatic drive

The United States is sending a team of senior envoys to Europe on Sunday for consultations on Iraq.

The hawkish Deputy Defence Secretary Paul Wolfowitz will spend two days holding talks in London and at the Nato headquarters in Brussels, before travelling on to Turkey - whose airbases could play a key role in any attack against Iraq.

Mr Wolfowitz will return to Washington on Tuesday

He will be accompanied by the State Department's Under Secretary for Political Affairs, Marc Grossman, who will go on to Cyprus, Greece, Germany, the Netherlands, France and Portugal after Mr Wolfowitz heads back to Washington on Tuesday.

Correspondents say the officials will be following up American requests to governments for military contributions in case of war, and seeking to build a more solid political coalition against Iraq.

The US has threatened to use force to disarm Iraq, which it alleges is in possession of weapons of mass destruction.

Freeze

On Saturday, UN weapons inspectors in Iraq mounted further searches for hidden weapons, including a visit to the "Mother of All Battles" military complex, just south of Baghdad.

Inspection teams also spent several hours at another complex nearby and went to a facility at Balad, some 90 kilometres (56 miles) north of the capital, where the Iraqis hold chemical warfare training for military personnel.

	:	Iraq spotlight
Click to see maps of Iraq's suspected weapons sites

The UN teams have so far visited sites with well-known involvement in Iraq's past weapons programmes, and places where equipment had been disabled or destroyed after UN inspections in the 1990s.

So far, access has been granted swiftly and all questions answered.

But our correspondent says the real test will come when the inspectors arrive at a facility which the Iraqis are not aware is on the UN's list for inspection.