By Simon Lewis

YANGON (Reuters) - Myanmar is investigating the death in custody of a 60-year-old Rohingya Muslim, the office of President Htin Kyaw said, as a security sweep in the country's northwest is increasingly beset by allegations of human rights abuses.

Security forces moved into northern Rakhine State after coordinated attacks on three border guard posts on Oct. 9 killed nine police officers.

The sudden escalation of violence in Rakhine state poses a serious challenge to the six-month-old government led by Aung San Suu Kyi, who was swept to power in an election last year but has faced criticism abroad for failing to tackle rights abuses against the Rohingya and other Muslims.

The government has said some 400 Rohingya militants with links to Islamists overseas were behind this month's attacks and later clashes in which five soldiers were killed. A group calling itself Al-Yakin Mujahidin claimed responsibility for the attacks in videos posted online.

UN officials are pressuring Myanmar to allow aid and observers into the area, where the majority of residents are stateless Rohingya Muslims.

Residents and human rights campaigners say security forces have killed civilians, arbitrarily detained residents, committed rape and burned houses.

Htin Kyaw's office said via its website that authorities had opened an investigation into the death of Khawrimular, who was detained on Oct. 14 on suspicion of involvement in the earlier attacks along with his three sons and two of his brothers.

Sources in the Maungdaw area of Rakhine told Reuters Khawrimular was a community leader and previously worked for international aid organizations in the area.

Soldiers interrogated Khawrimular and returned him to Kyeinchain police station, where the suspects were being held, on the morning of Oct. 17, said an official account published in the state-run Global New Light of Myanmar newspaper on Thursday.

"On the way back, the suspect grabbed a firearm from a soldier. Responsible personnel managed to subdue Khawrimular, but he lost consciousness as a result," the report said, adding that he died on the way to hospital.

Security forces have killed at least 33 alleged attackers since Oct. 9 and more than 50 people have been arrested. At least two other deaths in custody have been reported.

Eight Rohingya women told Reuters this week that they were raped by soldiers who entered the remote U Shey Kya village to conduct what authorities called a "clearance operation" on Oct. 19.

The 1.1 million Rohingya living in Rakhine face discrimination, severe restrictions on their movements and access to services, especially since inter-communal violence in 2012 that displaced 125,000 people.

The U.N.'s human rights envoy on Myanmar, Yanghee Lee, told reporters in New York on Thursday that she had heard "horrific stories" of abuses in Maungdaw.

"I have urged that there has to be complete access to this area and an impartial investigation needs to be conducted to verify, to explore the scope and nature and the cause of this recent attack," she said.

(Editing by Nick Macfie)