Four Chinese nationals have been hacked to death in Port Moresby, with one reportedly beheaded and the others dismembered in an attack condemned as "brutal and cowardly" Wednesday by Papua New Guinea Prime Minister Peter O'Neill.

O'Neill called for calm after the grisly murders, believed to have been committed with knives or swords in the Koki area of the Pacific nation's capital on Monday night.

"I condemn this brutal and cowardly attack on the four Chinese nationals," O'Neill said in a statement.

"I want to assure the government of China and relatives of those killed that police will get all the help necessary to track down and bring the perpetrators to justice."

The four -- three men and a woman -- were hacked and stabbed repeatedly by attackers who jumped a high fence outside the bakery they ran near the popular Koki market, according to media reports.

Radio New Zealand cited police as saying one was beheaded and the others were "chopped up", although this could not be independently confirmed.

O'Neill said it was a "heinous" crime and urged the business community, "especially those of Chinese and Asian origin", to remain calm and continue business as usual.

"Police have taken full control and an investigation is underway. Business should continue as normal," the prime minister said.

Chinese migrants first settled the Pacific islands in the 19th century but an influx of new migrants -- some illegal -- since the 1980s has seen them become the focus of political unrest.

Protest marches against relatively well-off Chinese business-owners in impoverished Port Moresby in 2009 descended into violence which saw two people killed.

Riots against Chinese traders erupted in the Solomon Islands and Tonga in 2006 over similar resentments.

Papua New Guinea recently passed harsh new laws reviving the death penalty as it grapples with a wave of violent crime, particularly against women, which has drawn international condemnation.

O'Neill described the new measures passed last month as "tough but necessary" in the face of rampant violent crimes including the beheading of one woman and torching alive of another as well as the gang-rape of two foreigners.