Gulf leaders have gathered in Saudi Arabia for a summit that is expected to see the end of a three-year embargo against Qatar by its neighbours.

Qatar's Emir Sheikh Tamim Al Thani was greeted by Saudi Crown Prince Mohammed bin Salman on arrival in al-Ula.

On Monday night, Saudi Arabia reopened its borders to Qatar and a US official said an agreement would be signed.

Saudi Arabia, the United Arab Emirates, Bahrain and Egypt cut ties with Qatar after alleging it supported terrorists.

The tiny, gas-rich state denied the accusation and rejected a list of demands issued by its neighbours at the start of the crisis, which included downgrading diplomatic ties with Iran and shutting down the Doha-based Al Jazeera network.

In recent months, the US has intensified pressure for a resolution of the dispute, insisting Gulf unity is necessary to isolate Iran.

The lifting of the embargo on Qatar has taken months of patient, painstaking diplomacy, mostly by Kuwait, but with increasingly urgent prodding from the White House as the Trump presidency draws to a close.

The three-and-a-half year "blockade" has been immensely costly to both Qatar's economy and to the notion of Gulf unity. Qataris will not forgive or forget in a hurry what they see as a stab in the back by their Gulf Arab neighbours.

But beyond the diplomatic rhetoric one country in particular - the UAE - has grave doubts that Qatar is actually going to change its ways. While Qatar denies supporting terrorism it has supported political Islamist movements in Gaza, Libya and elsewhere, notably the transnational Muslim Brotherhood which the UAE views as an existential threat to its monarchy.

Meanwhile, the embargo has, if anything, pushed Qatar closer to Saudi Arabia's ideological enemies: Turkey and Iran.

How did we get here?

The breakthrough was announced on TV on Monday night by Kuwaiti Foreign Minister Ahmad Nasser Al Sabah.

Mediation efforts led by Kuwait had achieved little until recently, but in the past few months there were growing signs of a resolution to the dispute.

The US has taken a more visible role and the US official said President Donald Trump's son-in-law and senior adviser, Jared Kushner, would attend the signing of the agreement on Tuesday.

Saudi Crown Prince Mohammed bin Salman said on Monday the summit was aimed at "reunification and solidarity in facing the challenges in our region", according to the official Saudi Press Agency.

A source involved in organising the summit said the Saudi decision to open its air space and land and sea borders was the confidence-building step needed to ensure the emir of Qatar's attendance, BBC Chief International Correspondent Lyse Doucet reports.

media captionQatar's foreign minister interviewed in 2018
In 2017, when the embargo was imposed, the emir said he would not travel to any country which restricted the entry of Qatari citizens.

"This is the biggest breakthrough we've had to date," a senior Trump administration official was quoted as saying by the Wall Street Journal.

"It doesn't mean they will love each other and be best friends, but it does mean they will be able to work together."

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