Four Kuwaitis have been arrested for comments made on Twitter and will be detained for 10 days for questioning, an Interior Ministry source said on Monday, after local media reported the detainees had criticised the country's ruler.

Kuwait, a U.S. ally and major oil producer, has been taking a firmer line on social media and has charged several citizens this year for comments made on the site which were deemed to be stoking tensions in the Gulf Arab state.

Twitter is extremely popular in the country.

"They were summoned and transferred to the prosecution. The public prosecution ordered a 10-day detention," the official said, declining to be named.

The source added that the four were arrested for comments made on Twitter but did not give any details.

Kuwaiti newspapers have reported that the four were arrested last week over comments deemed insulting to the emir, Sheikh Sabah al-Ahmad al-Sabah, who is described as "immune and inviolable" in the constitution.

This issue has become sensitive in Kuwait since demonstrations against new voting rules ordered by the emir last month ahead of Dec. 1 parliamentary elections.

The emir has said the changes were constitutional and were aimed at improving the voting system to help preserve national unity.

A prominent opposition politician, Musallam al-Barrak, has also been charged with comments deemed insulting to the emir made at an opposition rally in October.

Such direct public criticism is extremely rare in Kuwait and marked a shift in opposition rhetoric.

While public demonstrations about local issues are common in a state that allows the most dissent in the Gulf, Kuwait has avoided Arab Spring-style mass unrest that toppled three veteran Arab dictators last year.

But tensions have intensified between the elected parliament and opposition groups, and the hand-picked government in which ruling family members hold the top posts.