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 set off to visit 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) has 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 affected areas in order to provide humanitarian assistance and 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 fulfil 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."
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.
Suu Kyi, on a visit to Japan, was meeting the Myanmar diaspora on Wednesday and was set to meet Prime Minister Shinzo Abe in the afternoon.
The ambassadors of the United States, China, Britain and the European Union left the Rakhine capital, Sittwe, on Wednesday for the northern part of the state under military lockdown.
They were led by Nyi Pu, the Suu Kyi-appointed chief minister of Rakhine State. The list of participants reviewed by Reuters also included the top U.N. representative in Myanmar, Renata Lok-Dessallien, as well as envoys from several other countries, including India, Turkey and Indonesia.
The officials will visit the Maungdaw area, although the government has not provided a detailed itinerary.
The officials have privately expressed scepticism that the high-level diplomatic mission will tackle the concerns raised by the international community or gain thorough access and will be able to investigate abuses independently.
The U.N. Office for the Coordination of Humanitarian Affairs (UNOCHA) urged for aid agencies to be granted access to the 10,000 to 15,000 people thought to have been displaced by the latest violence in Rakhine State.
"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.
In a sign that the mission was carefully managed by the authorities, state media have been invited to film the visiting diplomats, but no international reporters were informed of the trip or allowed to join.
Rohingya sources from the area have echoed the concerns about independent access to witnesses, but said the diplomats were likely to visit villages where residents have told Reuters of rapes, destruction of homes and killings of civilians.