Kuwait announced on Thursday it is to expel a number of Iranian diplomats for alleged spying, in a fresh blow for Arab-Persian ties across the Gulf.

Sunni-ruled Bahrain, a fellow Gulf state and scene of protests led by its Shiite majority, has accused Shiite Iran of meddling in its affairs and elements of the Bahraini opposition of links with Tehran.

The unrest and charges of Iranian ties have raised concerns in the Sunni monarchies of the oil-rich Gulf, which sent a joint military force to Bahrain where security forces crushed an anti-regime protest movement on March 16.

Foreign Minister Mohammed al-Sabah told reporters that Kuwait is to expel an unspecified number of Iranian diplomats for alleged links to a spy ring working for Tehran, reportedly ever since the 2003 invasion of Iraq.

"There will be action against a group of Iranian diplomats ... They will be considered persona non grata and expelled from Kuwait," he said.

The foreign minister charged that the diplomats had proven links to the suspected spy ring, three members of which a Kuwaiti court condemned to death to Tuesday.

An Iranian foreign ministry source, quoted earlier by state news agency IRNA, denied any such link. "This claim is a lie and baseless. This issue has nothing to do with the Islamic Republic of Iran," the source insisted.

The three men condemned to death -- two Iranians and a Kuwaiti national -- were all serving in Kuwait's army when they arrested in May 2010. Iran at the time also strongly denied any involvement.

The court heard charges that the spy ring had passed on confidential military information, taken pictures of Kuwaiti military installations and spied for Iran.

The ruling "showed a conspiracy was being hatched against Kuwaiti political, economic and military security," by Iran, the foreign minister said.

"What we saw in the ruling has shocked us ... that there is a conspiracy network linked to official sides in the Islamic republic. As a result we have set up a foreign ministry crisis cell and recalled our ambassador" from Tehran.

Sheikh Mohammed said Iran's charge d'affaires at its embassy in Kuwait City was summoned and handed an official protest at the foreign ministry.

According to local media, the men confessed to photographing Kuwaiti and US military sites for Iran's elite Revolutionary Guard, although the defendants denied the charges and said confessions were extracted under torture.

Kuwait's Al-Qabas newspaper on Thursday said three Iranian diplomats were involved in the spy cell but the court could not prosecute because of their diplomatic immunity.

Iranian diplomats started to recruit members of the ring a decade ago, according to the daily which cited details of the court ruling.

It said cell members had during the 2003 US-led invasion of Iraq supplied Tehran with information on Kuwaiti army and coalition movements stationed in the emirate, used as a springboard for the campaign.

About 45,000 Iranians live and work in predominantly Sunni Kuwait, which also has a sizeable Shiite minority.

On the Gulf front, the United States has accused Iran of undermining peace and stability in the region by trying to advance Tehran's agenda in its pro-Western Arab neighbours across the waterway.

"We share the view that Iran's activities in the Gulf, including its efforts to advance its agenda in the neighbouring countries undermines peace and stability," US Secretary of State Hillary Clinton said earlier this month.

Tehran has publicly come out in support of Arab protest movements in Bahrain and Yemen. Its Defence Minister Ahmad Vahidi said kingdom had committed a "strategic and political" blunder that would cost its "legitimacy."