A number of Arab countries including Saudi Arabia and Egypt 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 denies.
The Saudi state news agency SPA said Riyadh had closed its borders, severing land, sea and air contact with the tiny peninsula of oil-rich Qatar.
Qatar called the decision “unjustified” and with “no basis in fact”.
The unprecedented move is seen as a major split between powerful Gulf countries, who are also close US allies.
It comes amid heightened tensions between Gulf countries and their near-neighbour, Iran. The Saudi statement accused Qatar of collaborating with “Iranian-backed terrorist groups” in its restive eastern region of Qatif and in Bahrain.
The diplomatic withdrawal was first put into motion by Bahrain, then Saudi Arabia, the United Arab Emirates (UAE), Egypt, Yemen, Libya’s eastern-based government and the Maldives all followed suit.
SPA cited officials as saying the decision was taken to “protect its national security from the dangers of terrorism and extremism”.
Saudi Arabia, the UAE and Bahrain have given all Qatari visitors and residents two weeks to leave their territory. The three countries have also banned their citizens from travelling to Qatar.
However, Saudi Arabia says it will still allow Qataris to take part in the annual Hajj pilgrimage to Mecca.
So far, there has been no sign of reciprocal moves by Qatar.
While the severing of ties was sudden, it has not come out of the blue, as tensions have been building for years, and particularly in recent weeks.
Two weeks ago, Egypt, Saudi Arabia, Bahrain and the UAE blocked Qatari news sites, including Al Jazeera. Comments purportedly by Qatari Emir Sheikh Tamim bin Hamad al-Thani criticising Saudi Arabia had appeared on Qatari state media.
The government in Doha dismissed the comments as fake, attributing the report to a “shameful cybercrime”.
Back in 2014, Saudi Arabia, Bahrain and the UAE withdrew their ambassadors from Qatar for several months in protest over alleged interference in their affairs.