YANGON - Myanmar has sent hundreds of soldiers to beef up security in northwestern Rakhine state after a recent spate of killings, military sources said on Friday,

fuelling fears of yet more violence and instability in the troubled region.

Muslim-majority northern Rakhine was plunged into violence last October when Rohingya Muslim insurgents killed nine police, setting off a brutal counteroffensive beset by allegations of rape, killings and torture by government troops.

Two military sources based in Rakhine told Reuters the army had sent troops to the state's north to "help tighten security" after seven Buddhists were found hacked to death in mountains near the town of Maungdaw last week.

The army dispatched about 500 soldiers to several towns near the border with Bangladesh on Thursday, including the towns of Buthidaung and Maungdaw, according to one of the military sources with direct knowledge of the matter.

"We have to increase security operations because the security situation has worsened — some Muslims and Buddhists have been killed by the insurgents," Rakhine State police chief Colonel Sein Lwin told Reuters.

The military spokesman and a spokesman for Myanmar's de facto leader, Aung San Suu Kyi, were not immediately available for comment.

Such a buildup raises fears of a fresh wave of violence after last year's operation in which security forces allegedly shot villagers at random, raped Rohingya women and burned homes.

UN expresses "major concern"

"This development, which reportedly took place yesterday, is a cause for major concern," said Yanghee Lee, the U.N. special rapporteur on the human rights situation in Myanmar.

"The government must ensure that security forces exercise restraint in all circumstances and respect human rights in addressing the security situation in Rakhine State," she said in a statement issued in Geneva.

United Nations investigators who interviewed some of the nearly 75,000 people who fled to neighbouring Bangladesh last year said troops probably committed crimes against humanity.

The government rejects the allegations and has refused to cooperate with a U.N. fact-finding mission to look into abuses in Myanmar.

Despite the massive counter-offensive last year, the government has accused the militants of continuing to run training camps in the mountains and killing alleged informants in the Muslim community.

The government blamed "extremists" for the killing of the seven Buddhists who, residents of the area believe, stumbled upon a camp for Rohingya militants.

That prompted security forces to hunt for the killers in an "intensive clearance operation", a military source told Reuters last week.

This week, the army has tightened security and the police force is on high alert, Rakhine police official Sein Lwin added.

Security forces have also strengthened border guard posts in the region, said Kyaw Swar Tun, the director of the general administration department based in the state capital, Sittwe.

Regional rights body ASEAN Parliamentarians for Human Rights voiced concern about the increased number of troops in Rakhine.

"Aung San Suu Kyi should call on all parties, including the Myanmar army, to take steps to de-escalate conflict in northern Rakhine State, rather than exacerbate it," a member of its board, Eva Kusuma Sundari, said in a statement.

About 1.1 million Rohingya live in Rakhine, but are denied citizenship and face curbs on movement and access to basic services. About 120,000 live in camps set up after deadly violence in 2012, relying on aid agencies for basic supplies.