YANGON —  A UN human rights envoy entered Myanmar for the first time in four years Sunday on a mission to uncover how many people were killed and detained since September's bloody crackdown on pro-democracy protesters.

Paulo Sergio Pinheiro, the UN's independent rights investigator for Myanmar, has said he is determined to gain access to the country's prisons and detention centres as part of an investigation into wide-ranging allegations of abuse committed by the military regime.

Pinheiro had been barred from the country since November 2003. He submitted a proposed itinerary to the ruling junta before arriving in the country for a five-day trip, but it was still being "fine-tuned,'' Aye Win, the UN spokesman in Myanmar, said Sunday evening.

"I hope I will have a very productive stay,'' Pinheiro told reporters after flying into Yangon, Myanmar's largest city, earlier in the day. He added, "I'm just very happy to be back here after four years.''

Pinheiro has a history of prickly relations with the ruling generals. He abruptly cut short a visit in March 2003 after finding a listening device in a room at a prison where he was interviewing political detainees. Later that year, he accused the junta of making "absurd'' excuses to keep political opponents in prison.

Accompanied by authorities, Pinheiro's first stop in Myanmar was the town of Bago, 80 kilometres north of Yangon, the UN said in a statement. Buddhist monasteries in Bago were among those targeted by the crackdown after monks joined anti-government street protests.

Pinheiro then returned to Yangon to meet officials at Shwedagon Pagoda, the country's most revered shrine and a flash point of unrest during the protests.

The junta, which has long been criticized for human rights abuses, has come under renewed international pressure since crushing the demonstrations. Myanmar authorities have said 10 people were killed when troops opened fire on peaceful protesters in Yangon on Sept. 26 and 27. Diplomats and dissidents, however, say the death toll was much higher and that an unknown number of people remain in custody.

Pinheiro cited unidentified sources as saying last month that between 30 and 40 monks and 50 to 70 civilians were allegedly killed.

Amnesty International submitted a letter Friday to Myanmar authorities expressing concern over "grave and ongoing human rights violations'' committed since the crackdown, including "widespread arbitrary detentions, hostage-taking, beatings and torture in custody and enforced disappearances.''

The London-based human rights group said about 700 political prisoners remain in custody. It demanded that Pinheiro be given full and unrestricted access to the country's detention centres.

Pinheiro has said he will abandon his current visit unless he gets full support from the junta.

"If they don't give me full co-operation, I'll go to the plane, and I'll go out,'' he said last month after getting the green light to return to Myanmar.

He said that since the crackdown he has received "worrying reports of death in custody, torture, disappearances, ill-treatment, and lack of access to food, water or medical treatment in overcrowded unsanitary detention facilities.''

The UN Human Rights Council condemned the crackdown at an emergency session Oct. 2 and urged an immediate investigation of the rights situation in Myanmar.

Pinheiro's trip comes three days after the departure of UN envoy Ibrahim Gambari, who attempted during a six-day visit to kick-start talks between the junta and the pro-democracy opposition.

As a result of Gambari's trip, detained opposition leader Aung San Suu Kyi was allowed to meet the leaders of her opposition party on Friday for the first time in three years. Suu Kyi said through a party spokesman she was "very optimistic'' about the prospects of dialogue with the government.

The regime cracked down on Suu Kyi's National League for Democracy party after it won elections in 1990. Instead of honouring the election results, the military stepped up a campaign of arrest and harassment of the party members, and eventually closed most of its offices.

Suu Kyi has been detained for 12 of the past 18 years, and continuously since May 2003.