Saudi Arabia, Egypt, Bahrain and the United Arab Emirates have cut diplomatic ties with Qatar, accusing it of destabilising the region.
They say Qatar backs terrorist groups including Islamic State (IS).
The Saudi state news agency SPA said Riyadh had closed its borders, severing land, sea and air contact with Qatar and largely isolating it.
It cited officials as saying it was to "protect its national security from the dangers of terrorism and extremism".
The unprecedented move is being seen as the most serious split yet between powerful Gulf countries, who are also close US allies.
The row comes two weeks after the same four countries blocked Qatari news sites. Controversial comments by Qatari emir Sheikh Tamim bin Hamad al-Thani, appearing to criticise Saudi Arabia,
appeared online.
The government in Doha dismissed the comments as fake, attributing the report to a "shameful cybercrime".
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