Saudi Arabia and several other Arab countries cut ties with Qatar on Monday, accusing it of supporting extremism.
Bahrain, the United Arab Emirates, Yemen, Libya and the Maldives joined Saudi Arabia and Egypt in severing relations with gas-rich Qatar. Riyadh has accused Doha of supporting groups, including some backed by Iran, "that aim to destabilise the region."
Qatar reacted by denying that it was providing any support for extremists and accused its Gulf neighbours of seeking to put the country under "guardianship."
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.
Here are the countries that reacted so far:
A senior Iranian official said on Monday the decision by some Gulf Arab states and Egypt to sever diplomatic ties with Qatar would not help end the crisis in the Middle East.
"The era of cutting diplomatic ties and closing borders ... is not a way to resolve crisis ... As I said before, aggression and occupation will have no result but instability," Hamid Aboutalebi, the deputy chief of staff of Iran's President Hassan Rouhani, said on Monday.
"What is happening is the preliminary result of the sword dance," Aboutalebi said, in a reference to US President Donald 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 extremists and singled out Iran as a key source of funding and support for militant groups.
US Secretary of State Rex Tillerson urged the Gulf Cooperation Council nations to sort out their differences.
Tillerson and Defense Secretary Jim Mattis said that they did not expect the decision by the Gulf countries to affect the fight against terrorism.
The region plays an important role for the US military in the fight against Daesh. Qatar is home to the Al Udeid Air Base, from where the United States carries out air strikes against militants in the region.
Turkey's Foreign Minister Mevlut Cavusoglu said on Monday he was saddened by the rift between Qatar and other Arab states, and called for dialogue to resolve the dispute.
"We see stability in the Gulf region as our own unity and solidarity," Cavusoglu said.
"Countries may of course have some issues, but dialogue must continue under every circumstance for problems to be resolved peacefully. We are saddened by the current picture and will give any support for its normalisation," Cavusoglu said.
Cavusoglu also said regional solidarity was needed more than ever in a time of deep instability and crises in various countries of the region.
"Turkey...is ready to do what is required of it to find a solution in the shortest period to this disagreement between fraternal countries," he said.
It is in Russia's interest to have a "stable and peaceful" situation in the Gulf, the Kremlin said on Monday, commenting on the Qatar crisis.
Moscow also said it hopes the current diplomatic row in the Gulf will not affect "the common determination and resolve" in the joint fight against "international terrorism," Kremlin spokesman Dmitry Peskov said.