Kuwait on Thursday expelled Iranian diplomats and closed some embassy missions after the emirate's top court convicted a "terror" cell of links to the Islamic republic, the foreign ministry said.
Kuwait, which has a sizeable Shia minority, is trying to mediate an end to the Gulf's worst crisis in years after Saudi Arabia and its allies cut ties with Qatar in June.
They severed relations with Qatar over accusations it is too close to Iran and funds extremists -- allegations denied by Doha.
A Kuwaiti supreme court ruling last month convicted 21 people of belonging to a cell that had been formed and trained by Iran's Revolutionary Guards. 
"Iranian sides helped and supported the cell members," Kuwait's foreign ministry said in a statement.
A senior government official, speaking on condition of anonymity, told AFP around 15 Iranian diplomats were expelled.
He declined to comment on whether the Iranian ambassador was among those ordered to leave.
Kuwait also ordered the closure of the Iranian "military, cultural and trade" missions, the official said.
Iranian authorities meanwhile summoned Kuwait's charge d'affaires, state-run IRNA news agency reported.
Kuwait's supreme court last month sentenced the mastermind of the cell to life in jail and condemned 20 others to various prison terms for alleged links with Iran and Lebanese Shia militia Hezbollah, and plotting terror attacks in Kuwait.
Regional tensions rise  
State Minister for Cabinet Affairs Sheikh Mohammad Abdullah al-Sabah, who is also acting information minister, confirmed Kuwait had taken diplomatic action against Iran.
"Following the supreme court ruling on the case... the government of Kuwait has decided to take measures, in accordance with diplomatic norms and the Vienna Convention, towards its relations with the Islamic Republic of Iran," he told AFP.
The latest development comes amid escalating tension in the Gulf, involving Qatar and four Arab countries -- Saudi Arabia, Bahrain, the United Arab Emirates and Egypt.
Kuwaiti authorities busted what they said was a "terrorism" cell with ties to Iran in August 2015 and seized large quantities of arms, ammunition and explosives. 
Cell members were convicted of working for Iran's Revolutionary Guards and Lebanon's Hezbollah. They were also convicted of smuggling explosives from Iran.
The only Iranian member of the cell was sentenced to death in absentia by the lower court early last year.
Kuwait's interior ministry on Tuesday said 14 convicts were on the run while the newspaper, Al-Seyassah, has said they had fled to Iran by sea.
The cell was convicted by a lower court early last year but acquitted on appeals several months later and set free.
The supreme court, whose rulings are final, overturned their acquittal on June 18 and sentenced them to between five and 15 years in prison. 
In March 2011, a Kuwaiti court condemned four people of forming a spy cell for Iran. That led to the expulsion of ambassadors by the two countries but they returned a few months later.
Kuwait has maintained good relations with Iran in recent years. 
In February, Iranian President Hassan Rouhani visited Kuwait City a month after the emirate launched a dialogue with Iran in a bid to normalise Gulf-Iran relations.