Bahrain, Egypt, the United Arab Emirates, Yemen and the Maldives also severed ties with gas-rich Qatar, which Riyadh accused of supporting groups, including some backed by Iran, "that aim to destabilise the region".

Qatar denied any support for extremists and accused its neighbours of seeking to put the country under "guardianship".

The crisis was likely to have wide-ranging consequences, for Qatar and its citizens as well as the Middle East and Western interests.

Qatar hosts the largest US airbase in the region, which is crucial in the fight against Islamic State group jihadists, and is set to host the 2022 football World Cup.

The dispute comes less than a month after US President Donald Trump visited Saudi Arabia and called for a united Muslim front 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 its official news agency.

The Gulf states and Egypt said they were severing ties and closing transport links with Qatar, which relies on imports from its neighbours.

- Qatar slams 'baseless claims' -

The Gulf states ordered Qataris to leave within 14 days and banned their own citizens from travelling to the emirate.

Saudi Arabia also closed its borders with Qatar, effectively blocking food and other exports and prompting shoppers to flood Doha's supermarkets.

In one store queues were up to 25-people deep as shoppers piled trollies and baskets high with supplies from rice to nappies.

"It's a cycle of panic and I needed to get pasta," said Ernest, a Lebanese who pushed two trollies.

Riyadh said its measures were the result of "gross violations committed by authorities in Qatar".

It accused Doha of harbouring "terrorist and sectarian groups that aim to destabilise the region including the Muslim Brotherhood, Daesh (IS) and Al-Qaeda".

Gulf states have long accused Qatar of supporting extremist groups, in particular the Muslim Brotherhood.

Riyadh also accused Doha of supporting Iran-backed "terrorist activities" in its east, as well as in Bahrain.

Saudi Arabia also shut the office of Qatar's Al-Jazeera global news channel. In the UAE, a subsidiary satellite network, beIn Sports, went offline.

Any suggestion Qatar is backing the agenda of Shiite-dominated Iran -- Sunni Saudi Arabia's regional arch-rival -- is especially sensitive, particularly after Trump's comments last month.

"The measures are unjustified and are based on false and baseless claims," Qatar said.

"The aim is clear, and it is to impose guardianship on the state," it said, insisting authorities would "take all measures necessary... to foil attempts to affect or harm Qatar's society and economy".

- Flights cancelled -

The dispute saw Qatari share prices close down 7.58 percent.

UAE carriers Emirates, Etihad, flydubai and Air Arabia, as well as Saudi Airlines announced the suspension of all flights to and from Qatar as of Tuesday morning.

Egypt said it would also suspend air links with Qatar from Tuesday and gave Doha's ambassador 48 hours to leave the country.

Qatar Airways suspended all flights.

In another blow to the carrier, Saudi authorities said the country's airspace would be closed to Qatari planes.

The Saudi-led coalition battling Iran-backed rebels in Yemen said it had expelled Qatar from the group for providing "support to (terrorist) organisations" there.

Gulf countries previously recalled their ambassadors from Qatar in 2014, ostensibly over its support for the Brotherhood, but Monday's moves go much further.

- Emboldened by Trump -

Saudi Arabia and its Gulf allies may have felt emboldened by Trump's visit, which saw the new president clearly align US interests with Riyadh and lash out at Iran.

US Secretary of State Rex Tillerson said he did not expect the announcement to have "any significant impact... on the unified fight against terrorism."

He encouraged Qatar and its neighbours to "sit down together".

Iran also urged Qatar and its neighbours to talk.

Its foreign minister, Mohammad Javad Zarif, spoke on the phone with his counterpart in Qatar, but also the foreign ministers of Algeria, Iraq, Lebanon, Oman, Tunisia and Turkey to discuss the "latest regional developments".

Turkey said it was ready to help defuse the row, with Foreign Minister Mevlut Cavusoglu lamenting a "development that really saddened all of us".

Russia said President Vladimir Putin spoke to Turkish counterpart Recep Tayyip Erdogan to discuss the dispute, adding both had "called all interested countries to dialogue".

Israeli Defence Minister Avigdor Lieberman told parliament the developments could herald a broad anti-terror alliance including the Jewish state.

- 'Unprecedented tensions' -

Early signs of an impending crisis emerged last month.

Doha said hackers were behind the release of false remarks attributed to Emir Sheikh Tamim bin Hamad Al-Thani published on the website of its national news agency.

The stories quoted him questioning US hostility towards Iran, speaking of "tensions" between Doha and Washington and speculating that Trump might not remain in power for long.

Doha denied the comments and denounced a "shameful cybercrime".

The crisis is the worst to hit Gulf Arab nations since the creation in 1981 of the Gulf Cooperation Council (GCC) grouping Saudi Arabia, Bahrain, the UAE, Kuwait, Oman and Qatar.

The Kuwaiti emir, Sheikh Sabah al-Ahmad Al-Sabah, called the Qatari ruler and urged him to observe restraint and "not to take any measure that could escalate" the situation.

The call came after Sheikh Sabah met an envoy from Riyadh.

Adam Baron, visiting fellow at the European Council on Foreign Relations, said the row represents "an unprecedented uptick in tensions within the GCC". (**)