Kuwait has protested to Lebanon over the alleged training by Hezbollah of 21 Shiites convicted last month on charges of forming a "terror cell" in the Sunni-majority emirate.
It demanded that pressure be put on the Shiite militant group, which has ministers in the Lebanese government, to ensure there was no repetition.
The protest follows Kuwait's expulsion of 15 Iranian diplomats on Thursday over Tehran's alleged links to those convicted.
The letter delivered by Kuwait's ambassador in Beirut late on Friday said the emirate's supreme court had found that "Hezbollah took part in contacts, coordinating meetings, paid funds and provided arms and military training in Lebanon" for the defendants.
It called on Lebanon to take "the necessary measures to curb these disgraceful practices" by Hezbollah as it is a partner in the government, ambassador Abdulaal al-Quenai told the official KUNA news agency.
Unlike neighbouring Sunni powerhouse Saudi Arabia, Kuwait maintains diplomatic relations with Shiite Iran.
But sectarian issues remain sensitive. Shiites make up around one third of the emirate's population.
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