Saudi Arabia, Egypt, Bahrain and the United Arab Emirates have cut diplomatic ties with Qatar, accusing it of destabilising the region.
The countries say Qatar is supporting terrorist groups including the Muslim Brotherhood.
The Saudi state news agency SPA said Riyadh had closed its borders, severing land, sea and air contact with Qatar.
It cited officials as saying it was to “protect its national security from the dangers of terrorism and extremism”.
Egypt has also closed its airspace and ports for all Qatari transportation, its foreign ministry said.
The United Arab Emirates has given Qatari diplomats 48 hours to leave the country. Abu Dhabi accuses Doha of “supporting, funding and embracing terrorism, extremism and sectarian organisations,” state news agency WAM said.
Bahrain’s state news agency said the country was cutting ties with Qatar because Doha was “shaking the security and stability of Bahrain and meddling in its affairs”.
The Saudi-led Arab coalition fighting Yemen’s Houthi rebels also expelled Qatar from its alliance because of Doha’s “practices that strengthen terrorism” and its support to groups “including al-Qaeda and Daesh [also known as the so-called Islamic State], as well as dealing with the rebel militias”, according to SPA.
Qatar has provided its warplanes to carry out air strikes against the Houthi rebels.
Qatar has so far made no public comments on the latest developments.
The row comes almost two weeks after controversial comments attributed to the Emir of Qatar, Sheikh Tamim bin Hamad al-Thani, appeared online.
The government in Doha has dismissed them as fake, attributing the report to a “shameful cybercrime”.