SYDNEY — Four Chinese nationals have been stabbed to death in Papua New Guinea in what the South Pacific island nation's prime minister on Wednesday condemned as a "brutal and cowardly" attack.

The victims, three men and one woman, were killed late Monday night at a shop they ran in the capital, Port Moresby, said Papua New Guinea national police spokesman Dominic Kakas. The assailants came in through the shop's back door, threatened the victims and then stabbed them with knives.

"I condemn this brutal and cowardly attack on the four Chinese nationals," Prime Minister Peter 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 victims are believed to be members of the same family, but police don't know where in China they are from or how long they had been living in Papua New Guinea, Kakas said.

Police have questioned several people, including workers at the shop, but have not made any arrests.

"They're not ruling out the possibility of the suspects being family or very close to those that were murdered," Kakas said.

In Beijing, Foreign Ministry spokeswoman Hua Chunying said China's ambassador to Papua New Guinea called senior officials in the country demanding swift justice, "and that Papua New Guinea take robust measures to ensure the safety of Chinese nationals and property." She said Chinese officials will provide all necessary support to the victims' families.

The nation's government recently proposed several tough new anti-crime measures in response to a surge in high-profile violent crimes. In February, a mob stripped, tortured and burned alive a woman accused of witchcraft in the city of Mount Hagan. Last year, police arrested 29 people accused of being part of a cannibal cult and charged them with the murders of seven suspected witch doctors.