By Louis Charbonneau

UNITED NATIONS (Reuters) - The United Nations on Thursday voiced concern about Bahrain's crackdown on the country's political opposition in what appears to be an escalating campaign against dissent in the Western-allied Gulf Arab kingdom.

A Bahraini court on Tuesday ordered the suspension of the main Shi'ite opposition group al-Wefaq and closed down its offices.

Al-Wefaq's lawyer, Abdallah al-Shamlawi, said the Bahrain administrative court had also set an October date for a hearing into dissolving al-Wefaq. The move came a day after authorities detained one of Bahrain's leading rights activists.

"The secretary-general (Ban Ki-moon) is concerned by recent actions of the Bahraini authorities seemingly aimed at restricting the country's political opposition," U.N. spokesman Stephane Dujarric told reporters.

Sunni Muslim-ruled Bahrain was rocked by mass protests by majority Shi'ite Muslims in 2011, when uprisings shook the Arab world, demanding a bigger role in running the small Gulf island, where the U.S. Navy's Fifth Fleet is based.

In addition to the move toward dissolving al-Wefaq, Dujarric cited the re-arrest of human rights activist Nabeel Rajab and the lengthening of the sentence of al-Wefaq leader Sheikh Ali Salman.

He said Ban was "dismayed about reports that suggest that human rights defenders and activists in Bahrain have been intimidated and even stripped of their citizenship for peacefully carrying out activities to promote human rights."

The U.S. State Department has also said it was deeply concerned over the al-Wefaq ruling. Bahraini opposition groups condemned the move.

Al-Wefaq, which held 18 out of 40 parliamentary seats in a 2010 election, pulled out of the assembly during the 2011 crackdown against mostly Shi'ite protesters demanding greater democracy. It boycotted the 2014 election saying it believed the vote would not be free or fair.

Rights groups have accused Bahrain of escalating a crackdown on the opposition over the past two years by jailing some activists and revoking citizenship of others and expelling them.

On Monday, police detained Rajab, one of the most prominent activists in the Arab world, nearly a year after he was freed by royal pardon.

His lawyer said on Tuesday Rajab was ordered detained for a week for questioning on charges of disseminating false news.

Last month an appeals court more than doubled a prison term imposed on Sheikh Ali Salman in June last year to nine years from four.

(Reporting by Louis Charbonneau; Editing by Tom Brown)