Joe Biden aims to 'keep the peace' as he flies to Belfast

Joe Biden boarded Air Force One with his sister Valerie and his son Hunter (left) for their flight to Belfast

US President Joe Biden said his priority was to "keep the peace" in Northern Ireland as he set off on Air Force One for a visit to Belfast.

He will arrive in the city tonight to mark the 25th anniversary of the Good Friday peace agreement.

The 1998 deal brought an end to the Troubles - the decades-long violent conflict in Northern Ireland in which thousands of people were killed.

A huge security operation is already in place in Belfast for Mr Biden's visit.

While he has praised what politicians did to secure peace in 1998, his visit is overshadowed by the fact that Northern Ireland's power-sharing government is not functioning.

It collapsed last year when the Democratic Unionist Party (DUP) - one of the biggest parties at Stormont - pulled out as part of a protest against post-Brexit trade rules for Northern Ireland.

Ahead of his arrival, Mr Biden said: "I look forward to marking the anniversary in Belfast, underscoring the US commitment to preserving peace and encouraging prosperity."

His visit to Belfast will be the first leg of a four-day stay in Ireland, during which he will discuss his Irish roots and meet Irish relatives.

Read more about Biden's visit

• Is this a missed opportunity for Stormont?

• Why Ireland is so important to Joe Biden

• Is the Good Friday Agreement working?

Mr Biden's trip comes two weeks after MI5 said the terrorism threat level in Northern Ireland had increased due to a rise in activity by dissident republicans.

During an illegal parade by dissident republicans in Londonderry on Monday petrol bombs were thrown at a police vehicle but the violence was confined to one area and ended a short time later.

On Tuesday, police found four suspected pipe bombs inside the grounds of the City Cemetery in Derry. They believe they were to be used in a planned attack on officers after Monday's parade.

The president's spokesman said Mr Biden was "more than comfortable making this trip" despite the terrorism threat.

On Monday the 80-year-old dropped another hint that he would seek re-election in 2024, saying he planned to run again but was "not prepared to announce it yet".

The president left Washington DC on Air Force One at about 10:00 EDT (15:00 BST) and will be met by PM Rishi Sunak when he lands at Belfast International Airport later.

Police have warned of traffic delays around the airport as officers facilitate the presidential motorcade.

Mr Biden will arrive as strong winds and heavy rain are sweeping across Northern Ireland, with a weather warning having been issued by forecasters.

Hundreds of extra police officers have been drafted into Belfast ahead of Joe Biden's arrival

He is expected to stay at a Belfast city centre hotel and, while his visit to Northern Ireland is much shorter than originally expected, Downing Street has dismissed suggestions it will be a "low-key" event.

The main event will be a speech at the new Ulster University campus in Belfast.

It is understood that he will use that to underscore the willingness of the US to help to preserve what he sees as the peace and prosperity gained since the Good Friday Agreement.

The president is also expected to talk about how the US administration can support Northern Ireland's economy.

And he is expected to meet the leaders of Stormont's five main political parties at some point during his brief time in the city.

There has been a huge police presence in the centre of Belfast since Monday afternoon and that will continue all through Tuesday and into Wednesday.

On Bedford Street police officers are patrolling at barricades close to the Grand Central Hotel but pedestrians can pass through and businesses in the area are operating as usual.

There has been some tension behind the scenes about the details of this visit.

Things have been strained between Downing Street and the White House in terms of what the president will do in Northern Ireland and the fact that he will not go to Stormont.

Rishi Sunak was keen to have his moment - this is, after all, a US presidential visit to the UK on his watch.

He will greet the president at the bottom of the steps of Air Force One tonight and there's a half-hour set aside for a one-to-one meeting on Wednesday - but not much more beyond that.

The PM will be not be at Ulster University with other politicians during the president's only public engagement in Belfast.

The fact that he will be doing other things elsewhere in Northern Ireland at the time is telling considering how important this visit is.

Michelle O'Neill, the vice-president of Sinn Féin, the largest party at Stormont, said Mr Biden's visit would be a special moment that "cements our close bonds of friendship".

Former Prime Minister Tony Blair said Mr Biden's visit could have a positive effect on restoring power-sharing at Stormont, but warned American influence on Northern Ireland should be handled with care.

"There's a difference between influencing and pressurising - one tends to be positive and the other can be negative," said Mr Blair.

"One thing I learned about the unionists is if you try to pressurise them to do something they are fundamentally in disagreement with it's usually futile pressure."

Newry man John Owen Finegan - a fourth cousin of Joe Biden - is hoping to meet him this week

Former Irish ambassador to the US Daniel Mulhall said Mr Biden would have preferred to have spoken to politicians at a functioning Stormont assembly, but said the message of the president's visit to Northern Ireland was essentially that America is here to help."

While Mr Sunak will not meet any of Northern Ireland's political leaders while he is in Belfast, Downing Street said this did not mean he had given up on getting the DUP back into power-sharing.

Mr Biden will leave Belfast on Wednesday afternoon to travel to the Republic of Ireland for three days of events in counties Louth and Mayo - where he has relatives - as well as Dublin.

He regularly speaks of his Irish heritage and had promised to visit the country during his presidency.

A US genealogist who researched his lineage had estimated he is "roughly five-eighths" Irish.

Among his great-grandparents was Edward Blewitt, an engineer and brickmaker who left the west coast town of Ballina in County Mayo in 1850.

He settled in Scranton in Pennsylvania as the devastating Irish potato famine was causing widespread starvation.

President Biden's maternal great-great-grandfather Owen Finnegan departed Carlingford in County Louth in the late 1840s to travel to America.

Joe Biden will visit the locations marked on this map during his four days in Ireland

Declan Harvey and Tara Mills explore the text of the Good Friday Agreement - the deal which heralded the end of the Troubles in Northern Ireland.

They look at what the agreement actually said and hear from some of the people who helped get the deal across the line.

Listen to all episodes of Year '98: The Making of the Good Friday Agreement on BBC Sounds.