Six Arab governments, including Egypt, cut diplomatic ties with Qatar early on Monday morning, with Yemen and Libya's eastern-based government among the latest to join the joint effort to isolate Doha.
The nations have cited Qatar's support for Islamist groups and alleged meddling in the affairs of other states.
The Gulf states of Saudi Arabia, Bahrain and the UAE were among those cutting relations with Qatar, further deepening a rift among members of the Gulf Cooperation Council, of which Qatar is a member.
The move began on Monday morning with Bahrain announcing that it was cutting diplomatic ties with Qatar and closing all transport links, according to Bahrain's state news agency. Bahrain cited alleged meddling in its internal affairs by Qatar.
Bahrain's action was followed by GCC ally Saudi Arabia, which said it was cutting ties on the grounds of "protection of national security", as announced by the Saudi state news agency.
The the SPA news agency said that Qatar “embraces multiple terrorist and sectarian groups aimed at disturbing stability in the region, including the Muslim Brotherhood, ISIS and Al-Qaeda, and promotes the message and schemes of these groups through their media constantly.”
The UAE, Yemen and Egypt then followed suit, with the eastern-based government of war-torn Libya finally joining the diplomatic action to isolate Qatar.
Egypt announced the closure of its airspace and seaports for all Qatari transportation to protect its national security, the foreign ministry said in a statement.
"Qatar's policy threatens Arab national security and sows the seeds of strife and division within Arab societies according to a deliberate plan aimed at the unity and interests of the Arab nation," the statement read.
Egypt accused the Gulf Arab state of supporting "terrorist" organisations, including the Muslim Brotherhood.
Yemen also cut ties with Qatar, saying it supported the decision by the Saudi-led coalition to end Qatar's participation in the war there. The government of President Abed Rabbo Mansour Hadi said it severed ties with Qatar in part over is support of extremist groups in Yemen "in contradiction with the goals announced by the countries supporting the legitimate government."
Saudi Arabia also said Qatari troops would be pulled from the ongoing war in Yemen.
The move to isolate Qatar was joined by Libya's eastern government later on Monday morning, according to its foreign minister, Mohamed Dayri. The eastern-based government is aligned with military commander Halifa Haftar, with whom Egypt has joined in recent military operations against Daesh terrorist camps in Libyan territory.
Daesh is among the terrorist groups that Qatar is accused of supporting across the region.
Bahrain and the UAE announced that their diplomatic staff had 48 hours to return from Qatar, meanwhile ordering Qatari diplomatic missions on their territory to cease operations.
Qatar hosts the sprawling Al-Udeid Air Base, which is home to the US military's Central Command and some 10,000 American troops, although no announcements have been made on the future of the US military presence there.
The move to isoloate Qatar has sparked fears of widespread travel disruption, with flights among the most seriously affected.
Saudi Arabia, Bahrain and the UAE announced that their citizens would be given 14 days to make arrangements before a ban would be imposed on travel to and from Qatar.
The Abu Dhabi-based airline Etihad says it has suspended flights to Qatar. Etihad said on its website Monday that "until further notice" its last flights would leave early Tuesday morning.
Qatar Airways said on its official website on Monday that it had suspended all flights to Saudi Arabia.
Qatar responded angrily to the joint action to isolate it, describing the move as "unjustified" and aimed at putting Doha under political "guardianship"..
While many Arab media outlets are reporting the developments as a response to Qatar's support for terroism, the Al-Jazeera news organization, which is owned by Qatar, has explained the dispute as stemming from a purported hack of Qatar's state-run news agency.
On 23 May, news emerged that the website and social media accounts of the Qatar News Agency had been hacked, allegedly carrying "fake news" reports relating to Qatari emir Sheikh Tamim bin Hamad Al-Thani.
This was followed by efforts by Egypt, Saudi Arabia and Bahrain to block all Qatar-based news agencies, including Al-Jazeera. Bahrain then complained that the Twitter account of its foreign minister had been hacked, with Qatar among the chief suspects.
Qatar has long faced criticism from its Arab neighbors over its support of Islamists. The chief worry among them is the Muslim Brotherhood, a Sunni Islamist political group outlawed by Egypta, Saudi Arabia and the UAE.
Gulf countries led by Saudi Arabia fell out with Qatar over its backing of former Egyptian President Mohamed Morsi, a Brotherhood member.
In March 2014, Saudi Arabia, the United Arab Emirates and Bahrain recalled their ambassadors from Qatar over the rift.
Eight months later, they returned their ambassadors as Qatar forced some Brotherhood members to leave the country and quieted others. However, the 2014 crisis did not see a land and sea blockade as threatened now.
In the time since, Qatar repeatedly and strongly denied it funds extremist groups.
However, it remains a key financial patron of the Hamas-controlled Gaza Strip and has been the home of exiled Hamas official Khaled Mashaal since 2012.
Western officials also have accused Qatar of allowing or even encouraging funding of Sunni extremists like al-Qaida's branch in Syria, once known as the Nusra Front.
The crisis also comes after US President Donald Trump's recent visit to Saudi Arabia for a summit with Arab leaders.
At that Saudi conference, Trump met with Qatar's ruling emir Sheikh Tamim bin Hamad Al Thani.
"We are friends, we've been friends now for a long time, haven't we?" Trump asked at the meeting. "Our relationship is extremely good."