Iran's foreign ministry on Friday slammed the imposition by Bahrain of jail terms on 13 leading Shia opposition figures in the Sunni-ruled Gulf kingdom, state television reported.

"The only solution to the crisis in Bahrain and to restore calm and stability is to respond to the legitimate demands of the population," it quoted the ministry's spokesman Ramin Mehmanparast as saying.

He added that for the Bahraini authorities to "pursue their current policies will only complicate the situation even further."

A Bahrain court on Tuesday upheld jail terms against the 13, including seven facing life in prison, on charges of plotting to overthrow the monarchy, lawyers said.

The opposition swiftly condemned the "vindictive" rulings and accused the court of staging "mock trials," the United States expressed concern, and London-based Amnesty International denounced the ruling as "outrageous."

On Wednesday, UN chief Ban Ki-moon urged the Bahraini authorities "to allow all defendants to exercise their right to appeal and to ensure that due process is observed."

Iran supports protests by Bahrain's Shia majority against the Saudi-backed Sunni minority, sparking a diplomatic crisis not only with the small Gulf kingdom but also with the area's economic powerhouse.

Bahrain came under strong criticism from international human rights groups over last year's crackdown on protests.

An international panel commissioned by King Hamad to probe the government's clampdown found that excessive force and torture had been used against protesters and detainees.

Bahrain, home to the US Fifth Fleet and strategically situated across the Gulf from Iran, has continued to witness sporadic demonstrations, mostly outside the capital Manama, since it crushed the protest movement in March last year.