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At least eight people have been killed in explosions at Myanmar's notorious Insein prison in Yangon.

Locals told ����ý Burmese two parcel bombs went off at the entrance to the jail on Wednesday morning, killing three prison staff and five visitors.

Insein prison is the country's largest jail housing about 10,000 prisoners, many of whom are political prisoners.

No group has claimed responsibility for the attack yet. Eighteen other people were injured, officials confirmed.

The authorities said the bombs had gone off in the prison's post room. Another bomb - which did not detonate - was later found there wrapped in a plastic bag.

All five of the visitors who were killed were women and relatives of prisoners, the authorities confirmed.

One was the mother of student leader Lin Htet Naing, alias James, who was arrested by Myanmar's military authorities last June. She had been visiting the prison to deliver a rice box to her son during the week of his court hearing.

Lin Htet Naing's father-in-law, Nay Win, said he'd been allowed to see her body in the morgue.

"I feel very sad. Her husband died quite a while ago and she was the only one who always went and dispatched food and necessities to her son," he said.

Lin Htet Naing, 35, has been arrested several times since coming to prominence as a student leader more than a decade ago. His arrest in June was for opposing last year's military coup, which toppled Aung San Suu Kyi's elected civilian government.

Lin Htet Naing's wife, Phyo Phyo Aung, a mother of two, has been on the run since she took part in the protests following the coup.

Insein prison is a vast, heavily-guarded complex on the outskirts of the former capital.

The century-old prison is infamous for its harsh conditions and inhumane treatment of prisoners, rights groups say.

However the junta faces stiff resistance in large parts of the country where there is an active guerrilla front known as the People's Defence Force (PDF).

In Yangon, there have been frequent bomb attacks, most of them small, since the military crushed the mass protest movement against its coup last year.

These have usually targeted individuals viewed as collaborating with the military - such as government officials, alleged informers and more recently air force pilots accused of taking part in aerial attacks on villages which are resisting military rule.

There have also been assassination attempts, drive-by shootings and, as attitudes have hardened, beheadings in rural areas which are blamed on both sides

The extent of deadly fighting and battles this year have been indicative of a civil war, observers say.