The killing of North Korea leader Kim Jong Un's estranged half- brother in Malaysia last week shows the brutality of isolated North Korea and its "terrorism tactics are getting bolder", South Korean Prime Minister Hwang Kyo-ahn said on Monday.

South Korean and U.S. officials believe Kim Jong Nam was killed by agents from the reclusive North, whose diplomats in Kuala Lumpur have sought to prevent an autopsy on the 46-year-old's body and demanded it be handed over.

Malaysian police said on Sunday they had arrested a North Korean man in connection with the murder of Kim Jong Nam, and that four other North Korean suspects had fled Malaysia on the day of the attack at Kuala Lumpur airport.

Hwang told a meeting of South Korea's National Security Council that it was nearly certain that North Korea was behind the killing of Kim Jong Nam, the eldest son of the late North Korean leader Kim Jong Il.

"The murder carried out in public at an international airport of a third country is an unforgivable and inhumane criminal act and clearly demonstrates the recklessness and brutality of the North Korean regime that will spare no avenues when it comes to perpetuating itself," Hwang said.

"As was seen in this case, the North Korean regime's terrorism tactics are getting bolder so we must be more vigilant about the possibility of terror by the North Korean regime against our government and people," Hwang said.

Kim Jong Nam had spoken out publicly against his family's dynastic control of the isolated, nuclear-armed nation.

Aside from the North Korean man, Malaysian authorities have also arrested a Vietnamese woman and an Indonesian woman in connection with the killing last Monday.

Malaysia's determination to carry out an autopsy and refusal to hand over the body directly to North Korean officials has strained diplomatic ties. Malaysia had been one of the few countries to maintain friendly relations with North Korea.

There is also speculation that China's patience with North Korea could be tested by the killing, as Kim Jong Nam had been living in the Chinese controlled territory of Macau.

Beijing, which is seen to be irritated by the North's repeated aggressive behaviour including two nuclear tests since the start of last year and a series of missile tests including a intermediate-range ballistic missile shot on Feb. 12, said on Saturday it had suspended all imports of coal from the North.

Coal exports to China are a vital source of revenues for the impoverished North.