SHAH ALAM, Malaysia—Two women on trial for killing the half-brother of North Korea’s ruler weren’t pranksters but trained assassins, Malaysian prosecutors said, urging a judge to reject defense arguments that the case should be summarily dismissed.

In closing statements Thursday for the prosecution phase of the trial, prosecutors said the defendants used “criminal force” and demonstrated training in attacking Kim Jong Nam, the estranged elder sibling of dictator Kim Jong Un, at Kuala Lumpur International Airport in February 2017. Kim Jong Nam died shortly afterward of poisoning by VX, a nerve agent banned by the United Nations as a weapon of mass destruction.

The two women—Doan Thi Huong of Vietnam and Siti Aisyah of Indonesia—are accused of having “common intention” with a group of North Koreans to kill Mr. Kim, who had been living in exile in Macau. The U.S. and South Korea say Mr. Kim was the target of a North Korean hit team working for Pyongyang. North Koreans accused of involvement fled Malaysia after the attack and none are standing trial.

After more than eight months of hearings, Judge Azmi Ariffin adjourned the trial to Aug. 16, when he will either dismiss the case entirely for lack of proof, freeing the women, or compel them to enter a defense. Both have pleaded not guilty and face death by hanging if convicted.

The prosecution has relied heavily on airport security footage showing the two women meeting a group of North Koreans at Malaysia’s busiest airport, then assaulting Mr. Kim near an airline kiosk, with at least one of them touching her hands to his face. Mr. Kim died shortly after of VX poisoning, a postmortem report showed. A chemical investigation found traces of VX on the clothing or hands of the women.