King Hamad bin Isa al-Khalifa has ratified amendments to Bahrain's constitution, with the monarch-led government hoping the new measures will defuse turmoil in the strategic Gulf state.

Opposition groups dismissed the extra powers given to parliament on Thursday as not enough, describing the moves as merely symbolic.

"The process of reform that marked the beginning of my tenure in power will not stop," said the Sunni monarch who succeeded to the throne of the Shia-majority kingdom in 1999.

"We hope in this important period that all forces and groups will assess their actions and join the process of progress and reforms," he said.

"Doors remain open to dialogue, which is aimed at achieving a national accord," he added, alluding to the Shia-led opposition that spearheaded months of pro-democracy protests that were crushed with deadly force in March last year.

According to the amendments, the king now has to consult the heads of the  elected parliament and the appointed Consultative Council before dissolving the legislature.

The elected chamber alone now has the right to vote to withhold its co-operation from the prime minister and refer its decision to the king, who then has the final choice on whether to dismiss the premier or keep him in post.

But the amendments fall far short of the demands of the opposition, which has been pushing for an elected government and the scrapping of the appointed  upper house.

Amendment criticisms

"The king remains the one to appoint and dismiss the government," said  Abdul Jalil Khalil, a leading member of the main Shia opposition group Al-Wefaq.

The opposition wants "a single-chamber parliament that is elected, just as  the constitution of 1973 stipulated", he added.

The consultative council was introduced in 2002, as part of a new constitution that turned Bahrain from an emirate into a kingdom.

It has been dismissed by the opposition as a move by the king to control parliament.

The opposition groups had opened a dialogue with Crown Prince Salman during the  pro-democracy demonstrations that broke out in February last year.

The talks broke down, however, when authorities turned to Bahrain's Gulf neighbours for military assistance to crush the protest movement the following month.

Despite the heavy-handed clampdown and a massive campaign of arrests and sackings targeting Shia, protesters have returned to the street, overshadowing the kingdom's hosting of the Formula One Grand Prix last month.

Amnesty International says nearly 60 people have been killed since the protests began last month.