The secretary-general of top opposition group in Bahrain - al Wefaq - Sheikh Ali Salman, and his political assistant Khalil al-Marzooq, were summoned on Tuesday by Bahraini interior ministry.

The secretary-general of top opposition group in Bahrain  - al Wefaq - Sheikh Ali Salman, and his political assistant Khalil al-Marzooq, were summoned on Tuesday by Bahraini interior ministry after a meeting they had with the U.S. Assistant Secretary of State for Democracy, Human Rights and Labor, Tomasz Malinowski, a spokesman of Wefaq told Reuters.

Subsequently, the visiting U.S. official who was subsequently ordered to leave the kingdom.

Bahrain ordered Malinowski to leave the kingdom for having "meetings with a particular party to the detriment of other interlocutors, thus discriminating between one people, contravening diplomatic norms and flouting normal interstate relations".

A Wefaq spokesman said Bahraini authorities intended to question Salman on Wednesday morning and no reason had been given for the move.

Wefaq is calling for a constitutional monarchy in the small Gulf Arab kingdom with a government chosen from within a democratically elected parliament.

Bahrain witnesses frequent unrest more than three years after the authorities cracked down the peaceful demonstrations for democratic reform.