SITTWE, Myanmar (Reuters) - A group of parliamentarians from the Association of Southeast Asian Nations (ASEAN) on Wednesday urged Myanmar to probe reports of human rights abuses in troubled Rakhine state, as top diplomats based in the country visited the area.

The U.S. State Department said the U.S. ambassador visited several villages in the northern part of the state as part of an international delegation and stressed the need for a thorough investigation into allegations of abuses and for aid workers to be allowed back into the area.

Troops have poured into northern Rakhine since militants believed to be Rohingya Muslims launched coordinated attacks on border posts on Oct. 9, killing nine police. The government says five soldiers and at least 33 alleged attackers have been killed in the military operation.

The territory has been cut off to aid workers and observers for more than three weeks. Residents and human rights advocates have said abuses by government forces have included summary executions, rape and setting fire to homes.

The government of Nobel Peace Prize winner Aung San Suu Kyi has denied any abuses have been committed.

The Rohingya, most of whom live in apartheid-like conditions, are seen by many Myanmar Buddhists as illegal immigrants from Bangladesh. Some 125,000 remain displaced and face severe travel restrictions in squalid camps since fighting erupted in Rakhine between Buddhists and Muslims in 2012.

ASEAN Parliamentarians for Human Rights (APHR) urged the Myanmar government to conduct a “thorough and impartial investigation into reports of abuses by security forces” against civilians in Rakhine.

It also called on the military to allow aid workers and journalists access to provide humanitarian help and to document developments.

“The reports coming out of Myanmar’s Rakhine State are alarming and demand a credible investigation ... All authorities must take urgent action to prevent further violations and fulfill their responsibility to protect the rights of all civilians,” said APHR Chairperson Charles Santiago, a member of the Malaysian parliament.

“We remain deeply concerned, however, that as a result of the lack of government oversight of security forces, effective systems are not in place to protect civilians or support their chance of seeing justice served.”

In Washington, U.S. State Department spokesman John Kirby said the U.S. ambassador to Myanmar, Scot Marciel, visited several northern villages where the Oct. 9 attacks took place with a delegation that included the U.N. resident coordinator and other chiefs of mission.

“The visit was an initial step in what we hope will be a continued assessment of the situation in the area by the government and by the international community,” Kirby said.

“The ambassador took the opportunity to stress to government officials accompanying the delegation that a thorough investigation into allegations of abuse, protection of all residents, restoration of full humanitarian access are necessary,” he told a regular news briefing.

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The military operation has sharpened the tension between Suu Kyi’s six-month-old civilian administration and the army, which ruled the country for decades and retains key powers, including control of ministries responsible for security.

The delegation that visited Rakhine state included ambassadors from China, Britain and the European Union as well as the United States.

The tour was led by Nyi Pu, the Suu Kyi-appointed chief minister of Rakhine State.

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In a sign that the mission was carefully managed by the authorities, state media was invited to film the visit but no international reporters were allowed to join.

The United Nations Office for the Coordination of Humanitarian Affairs (UNOCHA) urged access for aid agencies to 10,000-15,000 people thought to have been displaced by the latest violence.

“Essential, life-saving humanitarian activities have been suspended for more than three weeks now, and they need to be resumed as soon as possible,” said UNOCHA official Pierre Peron.