The first alert, at Belfast's international airport, where Bush's plane was to arrive at around 6:00 pm (1700 GMT), was lifted after a search of the site.

Two other alerts, one at Belfast's domestic City Airport, the other on a main route between the international airport and the town, were still in force, with the Sydenham bypass on the highway to the city airport blocked off and police carrying out inquiries.

A terminal at the City Airport was evacuated as a precaution.

Security was high in Belfast for the visit, with "hundreds of police" being deployed, a government source said.

The Northern Irish police said they were "working in the closest possible cooperation with the US secret service and the United Kingdom government during the visit."

A reliable source said there would be "ring of steel" for a two-mile radius around Hillsborough Castle, 10 kilometres (six miles) south of Belfast, where Bush and Blair were to meet, referring to police blockades of the area.

He said the bomb threats were "clearly designed to be disruptive and as troublesome as possible" and could be from Northern Ireland factions. But it was not clear who had made the threats.

In their third summit in just over three weeks, Bush and Blair were set to discuss the Iraq war and the future of the country after the expected fall of President Saddam Hussein, as well as the Middle East and Northern Ireland peace processes.

Thousands of anti-war protesters were expected to demonstrate at Hillsborough Castle.

There is expected to be a pro-war march at City Hall, in favor of local people fighting in Iraq.

SPACE.WIRE