Bahrain had witnessed mass protests inspired by the uprisings in Egypt and Tunisia [GALLO/GETTY]

Bahrain's monarch Hamad bin Isa Al Khalifa has said that a foreign plot against his kingdom had been foiled and thanked troops brought in from neighbouring countries to help end increasing unrest after weeks of protests.

State news agency BNA quoted Khalifa as telling the troops on Sunday night: "An external plot has been fomented for 20 to 30 years until the ground was ripe for subversive designs ... I here announce the failure of the fomented plot."

King Hamad told the forces that if such a plot succeeded in one Gulf Arab country, it could spill into neighbouring states, BNA said.

Bahrain had witnessed anti-government protests for weeks before security forces, bolstered by troops from other Gulf countries, smashed a protest site in the capital Manama and drove protesters off the streets.

The ferocity of the crackdown last week stunned Bahrain's majority Shias, the main force of the protests.

It also angered Iran, which is the region's non-Arab Shia power.

Iran, which supports Shia groups in Iraq and Lebanon, has complained to the United Nations and asked neighbours to join it in urging Saudi Arabia to withdraw forces from Bahrain.

Sunni rulers of Gulf states are concerned that Iran will gain more influence in the oil-rich region because of Bahrain's unrest.

King Hamad's announcement came after a day of tit-for-tat diplomatic expulsions between the Gulf island kingdom and Iran.

In a sign of rising tensions between the countries, Bahrain expelled Iran's charge d'affaires on Sunday, accusing him of contacts with some opposition groups, a diplomatic source said.

And Iran expelled a Bahraini diplomat in response. Bahrain has also said previously that it arrested opposition leaders for dealing with foreign countries.

Bahrain is home to the US navy's 5th fleet.