In a city that has known decades of bitter fighting between Catholics and Protestants, a police spokesman said the threats were "a disruption."

But he added: "These calls have to be taken seriously for everyone's protection and security."

It was not clear who had made the threats, but doubts have already been expressed about Bush's visit to the troubled region.

Gerry Adams, head of the republican Sinn Fein party, criticized Monday what he called the insensitivity of holding a war summit which discusses the Northern Ireland peace process "in the margins."

Adams, whose Sinn Fein opposes the war in Iraq, said it appeared the peace process was being used as a "stage or as a prop."

"We would be wrong not to point it out ... the insensitivity of having a war summit, of having a war summit which then discusses peace in the margins, of having a war summit which appears to be trying to use the Irish peace process as a stage or as a prop," Adams told Irish radio.

Thousands of anti-war protesters were expected to demonstrate at Hillsborough Castle, the site 10 kilometres (six miles) south of Belfast where the summit is to be held.

Meanwhile, there is expected to be a pro-war march at City Hall, in favor of local people fighting in Iraq, with the head of the province's largest Protestant party, attending.

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

Another alert at Belfast's domestic City Airport was lifted but a third on a main route between the international airport and the town, was still in force by early afternoon, with the Sydenham bypass heading 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 government source said security was helped by the fact that Bush's entourage will be smaller than the number of people who came from Washington when then-president Bill Clinton visited in 1995 and 1998 since the Bush trip is only a 24-hour affair and limited to one site.

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.

SPACE.WIRE