Security has been increased at the morgue where the body of North Korean leader Kim Jong-un's half brother is being held after attempts were made made to break in, Malaysian police said.

Kim Jong-nam's body is being held, as investigators attempt to establish what caused him to die from a seizure en route to hospital after complaining that a woman had sprayed chemicals on his face at Kuala Lumpur international airport.

Malaysian police chief Khalid Abu Bakar said authorities knew the identity of the break-in suspects, but would not be pushed to tell reporters whether they were North Korean.

“We know who they are. No need to tell you,” he told a news conference.

The break-ins have nonetheless fuelled speculation that North Korea played a role in his death. South Korean intelligence agencies have already accused the secretive state of assassinating Mr Kim.

They came as North Korean diplomats urged for the immediate release of “innocent females” arrested in connection with the 46-year-old's death.

The women, one Vietnamese and the other Indonesian, were detained last week after CCTV footage appeared to show them approaching Mr Kim in the airport and rubbing a toxic chemical in his face with their bare hands.

Some local media outlets have claimed that the pair were duped into playing a role in the killing after being told it was a television prank.

But Chief Khalid disputed this. He said the suspects were “trained” and had even practiced the attack at different public places, including a major mall in downtown Kuala Lumpur.

“Yes, the two female suspects knew that the substance they had was toxic," he said. "We don’t know what kind of chemical was used. They used their bare hands.”

He added that they were instructed to wash their hands afterwards.

Investigators also want to speak with a North Korean embassy worker and an employee of a North Korean airline as part of their investigation, Chief Khalid said, adding that both are thought to still be in Malaysia.

“They've been called in for assistance. We hope the embassy will cooperate with us and allow us to interview them quickly or else we will compel them to come to us,” he said.

Four people have so far been arrested in connection with Mr Kim's sudden death: a Vietnamese woman, an Indonesian woman, a Malaysian man and a North Korean man.

Mr Khalid said police “strongly believed” four suspects who fled from Malaysia on the day of Kim’s death had gone to the North Korean capital Pyongyang.

North Korea’s ambassador to Malaysia has meanwhile insisted that the man who died in the airport is not Kim Jong-un's brother and that Malaysia’s investigation into the death cannot be trusted.

Inside the daily life in North Korea Show all 19 1 /19 Inside the daily life in North Korea Inside the daily life in North Korea People reading a newspaper at the metro station Inside the daily life in North Korea Thoughts of the leaders on the tram. They have about a dozen of these on every tram, all with different thoughts Inside the daily life in North Korea Young people training for a big upcoming festival Inside the daily life in North Korea People at the Pyongyang's annual marathon Inside the daily life in North Korea Many stars on one of the trolleys in Pyongyang Inside the daily life in North Korea An intimidating poster in a primary school in North Korea. Inside the daily life in North Korea Solar panels installed on a street lamp. Inside the daily life in North Korea A poster on the window next to one of the venues we visited in Pyongyang Inside the daily life in North Korea Kids playing football next to the Arch of Triumph. After a while tourists were allowed to join, so some of us did Inside the daily life in North Korea Class in an educational center in Pyongyang (where people over 17 years old can attend any classes they choose after school, for free) Inside the daily life in North Korea People waving at me during the Pyongyang marathon Inside the daily life in North Korea People having a great time dancing at a public park Inside the daily life in North Korea A metro driver in a metro station in Pyongyang Inside the daily life in North Korea Fireworks to mark the birthday of the Eternal President Kim Il Sung on our last night in Pyongyang Inside the daily life in North Korea My wonderful tour guide at a public park Inside the daily life in North Korea One of the parks in Pyongyang Inside the daily life in North Korea A person rowing some boats for the day at a river in Pyongyang Inside the daily life in North Korea The National War Museum Inside the daily life in North Korea Public park in Pyongyang

A post-mortem examination was carried out at a hospital in Kuala Lumpur last week despite objections from diplomats at the North Korean embassy and their calls for the body to be released immediately. The autopsy was reportedly inconclusive.

Mr Kim, who was estranged from his younger brother and lived with his wife and two children in Macau on the south coast of China, had previously spoken out against his family’s dynastic control of North Korea.

The estranged, exiled half-brother of the North Korean leader is said to have been “paranoid” before his death, fearing that his brother would order his assassination, or that someone would try to kill him in a bid to impress the North Korean leader.

“He was afraid,“ a close friend of his told the The Guardian. ”It wasn’t an all-encompassing fear but he was paranoid. He was a politically important person. He was worried. Of course he was worried.