Iran's Supreme Court has upheld the death sentence for two men convicted of killing three Sunnis with the help of a British spy, a news agency reported on Monday, the latest move in worsening ties between the two countries.

The men, members of Komeleh Kurdish terrorist group, shot dead a son of Marivan's Friday prayer leader, along with two of his friends, in July 2009 with the support of the British intelligence service, the semi-official Mehr news agency reported.

The Mehr report said the court had found the two guilty of Moharebeh, an Islamic term meaning enmity with God, a capital crime that carries the death sentence.

Like neighboring Iraq and Turkey, Iran has a large Kurdish minority, mainly living in the west and northwest. Iran is a mainly Shi'ite Muslim country while most Kurds are Sunni Muslims.

According to Iran's state-run English Press TV, the men told a Revolutionary Court in late 2010 that a British agent had offered them a significant sum of money, British residency permits and other incentives to carry out the assassination in a west Iran city.

When lower courts impose a death sentence, the case goes to the Supreme Court to review the sentence.

Britain has been a focus of Iranian ire in the past months over steps to tighten sanctions over Tehran's disputed nuclear program.

Protesters stormed and ransacked Britain's two diplomatic compounds in Tehran in December, after London imposed financial sanctions following a United Nations report suggesting Iran was pursuing nuclear weapons.

The embassy attack pushed ties between Tehran and London to their lowest ebb since Iran severed diplomatic relations in 1989 over the publication of Salman Rushdie's book The Satanic Verses.

The head of the British Broadcasting Corporation said on Sunday that Iranian authorities were increasingly arresting and threatening the families of BBC journalists working outside Iran to force them to quit its Persian news service.

In a separate report Monday, Mehr said Iranian authorities had arrested several people who had cooperated with BBC's Persian-language service inside the country.

The report did not make clear how many people had been arrested but said they had been active in collecting news and other tasks since 2009.

Mark Thompson, director-general of the BBC, has denounced what he said was an attempt to put pressure on those who work for BBC Persian outside Iran, by targeting family members who still live inside the country.

Western countries suspect Iran's uranium enrichment activities have military aims. Tehran says they are for peaceful electricity generation.

(Editing by Alessandra Rizzo)