Bahrain, the United Arab Emirates, Yemen and the Maldives joined Saudi Arabia and Egypt in severing relations with gas-rich Qatar, with Riyadh accusing Doha of supporting groups, including some backed by Iran, "that aim to destabilise the region".
Qatar reacted with fury, denying any support for extremists and accusing its Gulf neighbours of seeking to put the country under "guardianship".
The crisis was likely to have wide-ranging consequences, not just for Qatar and its citizens but around the Middle East and for Western interests.
Qatar hosts the largest US airbase in the region, which is crucial to operations against Islamic State group jihadists, and is set to host the 2022 football World Cup.
Monday s announcement came less than a month after US President Donald Trump visited Saudi Arabia to cement ties with Riyadh and called for a united front among Muslim countries against extremism.
It also followed weeks of rising tensions between Doha and its neighbours, including Qatari accusations of a concerted media campaign against it and the alleged hacking of the Qatar News Agency.