Amnesty International said on Friday it had cancelled a visit to Bahrain after the Gulf state imposed restrictions on groups trying to monitor reforms including the handling of protests.
Amnesty, Human Rights Watch and another group had said Bahrain's Human Rights and Social Development Ministry informed them this week of the new rules limiting them to five-day trips on visas that must be arranged through a Bahraini sponsor.
The Bahraini government issued a statement expressing regret at the cancellation of the visit, saying that "Amnesty has chosen to put its objections to Bahrain's visa regulations before its work to promote and protect human rights."
The UN investigator into torture has also postponed a visit, the UN human rights office in Geneva saying that Bahrain had formally requested a delay until July.
"Regrettably we have cancelled the fact-finding visit to Bahrain ... as the new five-day limit imposed by the Bahraini authorities for visits by international human rights organisations is a serious impediment," Hassiba Hadj Sahraoui, a regional Amnesty deputy director, said in a statement.
"The Bahraini authorities have repeatedly stated their commitment to undertake human rights reform and to cooperate with human rights organisations. These new restrictions contradict such commitment," she said.
Bahrain, a U.S. ally ruled by the Sunni Muslim Al Khalifa family, has been under Western pressure to improve its rights record and institute political reforms after it crushed a pro-democracy uprising last year, imposing a period of martial law.
The government said on Thursday it would need up to 20 more days to complete its plans for implementing the recommendations of the Bahrain Independent Commission of Inquiry (BICI), which issued a damning report in November.
The BICI said protesters, who come mainly from the majority Shi'ite population, had undergone systematic torture to force confessions that were used in military trials.
The country remains in turmoil as clashes between youths and riot police occur daily in Shi'ite neighbourhoods and the banking and tourism-based economy, already weakened by the world financial crisis, struggles to pick up.
The new rules follow an Interior Ministry announcement that it would tighten tourist visa regulations after Western activists took part in anti-government demonstrations last month marking the first anniversary of the Feb. 14 uprising.
Bahrain is due to host the Formula One grand prix in April.
Washington, whose Fifth Fleet is based in Manama, and former colonial power Britain have pressed Bahrain to ensure peaceful protests are allowed. Police allowed the main parties to hold a rally in the capital this week.
Opposition parties want a move to full-scale parliamentary democracy, in which the elected chamber has full legislative powers and can form cabinets. The government has given parliament more powers of scrutiny over budgets and ministers.