Image caption North Korea has been accused of serious human rights violations

North Korean defectors have been giving international investigators harrowing accounts of brutality and abuse in the country's prisons.

A UN panel has begun interviewing witnesses in the South Korean capital, Seoul, and will move onto Japan.

One of the first witnesses to testify spoke of how he had been forced to watch the execution of his mother.

The North has been accused of human rights violations, including torture, abductions and running prison camps.

The UN named three panel members of this first-ever human rights investigation into North Korea in May.

'Finger chopped'

Shin Dong-hyeok was an early witness before the panel. Born in a North Korean prison camp, he told the inquiry he was forced to watch the execution of his mother and brother, after they had attempted to escape the camp.

He said that guards chopped off his finger after he dropped a sewing machine.

Another former inmate, Jee Heon-a, described how a mother had been forced to kill her own baby.

The hearings will last until Saturday, and more than 30 witnesses are expected to testify. They are taking place in Seoul and Japan because investigators were reportedly refused permission to enter the North.

Retired Australian judge Michael Kirby is chairing the inquiry, working with UN special rapporteur Marzuki Darusman and human rights activist Sonja Biserko.

It was set up to investigate "the systematic, widespread and grave violations of human rights" in North Korea, the UN said in a statement in May.

Violations to be investigated "are those pertaining to the right to food, those associated with prison camps, torture and inhuman treatment, arbitrary detention, discrimination, freedom of expression, the right to life, freedom of movement, and enforced disappearances, including in the form of abductions of nationals of other states", it said.

The inquiry will also look at who is accountable for any violations among state institutions and officials, reports the BBC's Lucy Williamson in Seoul.

More than 30 witnesses will be interviewed in five days of hearings, including North Koreans who have recently fled the country.

Commissioners will also meet government officials and campaign groups, and interview witnesses in Japan, before compiling a final report early next year.

The panel is due to present its report to the UN Human Rights Council in March 2014.