Saudi Arabia, Egypt, Bahrain, the United Arab Emirates and Yemen have cut diplomatic ties with Qatar, accusing it of destabilising the region.
They say Qatar backs militant groups including so-called Islamic State (IS) and al-Qaeda, which Qatar has denied.
The Saudi state news agency SPA said Riyadh had closed its borders, severing land, sea and air contact with the tiny peninsula of Qatar.
Qatar called the move “unjustified” with “no basis in fact”.
The unprecedented move is being seen as a significant split between powerful Gulf countries, who are also close US allies.
It comes in the context of increased tensions between Gulf countries and their near-neighbour Iran. The Saudi statement accused Qatar of collaborating with Iranian-backed militias.
What has happened?
The diplomatic withdrawal was put into motion by Bahrain then Saudi Arabia early on Monday. Their allies swiftly followed.
SPA cited officials as saying the decision was taken to “protect its national security from the dangers of terrorism and extremism”.
The three Gulf countries have given Qatari nationals two weeks to leave their territory.
While the severing of ties with Qatar was sudden, it has not come out of the blue, as tensions have been building over recent years, and particularly in recent weeks.
Two weeks ago, the same four countries blocked Qatari news sites, including Al Jazeera. Controversial comments purportedly by Qatari emir Sheikh Tamim bin Hamad al-Thani criticising Saudi Arabia appeared on Qatari state media.
The government in Doha dismissed the comments as fake, attributing the report to a “shameful cybercrime”.
Qatar says news agency was hacked
More broadly, there are two key factors driving Monday’s decision: Qatar’s ties to Islamist groups, and the role of Iran, Saudi Arabia’s regional rival.
While Qatar has joined the US coalition against IS, the Qatari government has been forced to repeatedly deny accusations from Iraq’s Shia leaders that it provided financial support to IS.
However, wealthy individuals in the emirate are believed to have made donations and the government has given money and weapons to hardline Islamist groups in Syria. Qatar is also accused of having links to a group formerly known as the Nusra Front, an al-Qaeda affiliate.
The SPA statement accused Qatar of backing these groups, as well as the widely-outlawed Muslim Brotherhood, and that it “promotes the message and schemes of these groups through their media constantly”.
What has been the reaction?
Qatar, which is due to host the football World Cup in 2022, was critical of the decision, in comments broadcast on Al Jazeera.
“The measures are unjustified and are based on claims and allegations that have no basis in fact,” Al Jazeera quoted the foreign ministry as saying. It said the decisions would “not affect the normal lives of citizens and residents”.
US Secretary of State Rex Tillerson, speaking in Sydney, urged the countries to resolve their differences through dialogue.
“I do not expect that this will have any significant impact, if any impact at all, on the unified fight against terrorism in the region or globally,” he added.
Qatar’s stock market plunged in early trading on Monday.