Libya, Saudi Arabia, Yemen, Egypt, the United Arab Emirates, Bahrain and the Maldives have severed their ties with Qatar, accusing it of supporting terrorism and opening up the worst rift in years among some of the most powerful states in the Arab world.
Iran - long at odds with Saudi Arabia and a behind-the-scenes target of the move - immediately blamed US President Donald Trump for setting the stage during his recent trip to Riyadh.
The six countries have already long resented Qatar's support for Islamists, especially the Muslim Brotherhood which they regard as a dangerous political enemy.
Monday's co-ordinated move created a dramatic rift among the Arab nations, many of which are in OPEC.
Announcing the closure of transport ties with Qatar, the Gulf states gave Qatari visitors and residents two weeks to leave.
Oil giant Saudi Arabia accused Qatar of backing militant groups - some backed by regional arch-rival Iran - and broadcasting their ideology, an apparent reference to Qatar's influential state-owned satellite channel Al Jazeera.
"(Qatar) embraces multiple terrorist and sectarian groups aimed at disturbing stability in the region, including the Muslim Brotherhood, ISIS (Islamic State) and al-Qaeda, and promotes the message and schemes of these groups through their media constantly," Saudi state news agency SPA said.
It accused Qatar of supporting what it described as Iranian-backed militants in its restive and largely Shi'ite Muslim-populated Eastern region of Qatif and in Bahrain.
Qatar said it was facing a campaign aimed at weakening it, denying it was interfering in the affairs of other countries.
"The campaign of incitement is based on lies that had reached the level of complete fabrications," the Qatari foreign ministry said in a statement.
Iran saw America pulling the strings.
"What is happening is the preliminary result of the sword dance," Hamid Aboutalebi, deputy chief of staff of Iran's President Hassan Rouhani, tweeted in a reference to Trump's recent visit to Saudi Arabia.
Trump and other US officials participated in a traditional sword dance during the trip in which he called on Muslim countries to stand united against Islamist extremists and singled out Iran as a key source of funding and support for militant groups.
US Secretary of State Rex Tillerson told reporters in Sydney the spat would not effect the fight against Islamist militants and that Washington has encouraged its Gulf allies to resolve their differences.
Watch: Tillerson calls for dialogue in Qatar rift
"I do not expect that this will have any significant impact, if any impact at all, on the unified - the unified - fight against terrorism in the region or globally," Tillerson told reporters in Sydney after meetings between Australian and US foreign and defence ministers.
Tillerson urged the Gulf Cooperation Council nations to instead sort out their differences.
A split between Doha and its closest allies can have repercussions around the Middle East, where Gulf states have used their financial and political power to influence events in Libya, Egypt, Syria, Iraq and Yemen.
The economic fallout loomed immediately, as Abu Dhabi's state-owned Etihad Airways, Dubai's Emirates Airline and budget carrier Flydubai said they would suspend all flights to and from Doha from Tuesday morning until further notice.
Qatar Airways said on its official website it had suspended all flights to Saudi Arabia.
The diplomatic broadside threatens the international prestige of Qatar, which hosts a large US military base and is set to host the 2022 World Cup. It has for years presented itself as a mediator and power broker for the region's many disputes.
'Serious development': expert
Dr Amin Saikal, from Centre for Arab and Islamic Studies said the rift could undermine the aim of the Gulf Cooperation Council, which was about cooperation between the Sunni-majority nations.
"It is a major and serious development," he told SBS World News.
"What Saudi Arabia, United Arab Emirates, Bahrain and Egpyt have done is to isolate Qatar as much as possible and therefore put a lot of pressure on Qatar.
"That will affect not only Qatari Airlines but also the Qatari broadcasting network that is Al Jazeera."
Dr Saikal said the rift's origins lay in the Arab Spring when the Muslim Brotherhood came into power.
