Footage from an airport security camera showed Vietnamese defendant Doan Thi Huong "practicing" wiping the face of a stranger two days before she allegedly took part in the murder of Kim Jong Nam using a similar technique, police told a Malaysian court on Tuesday.

Azirul Nizam Che Wan Aziz, the investigating officer, said he reviewed the CCTV footage taken at Kuala Lumpur International Airport budget terminal on Feb. 11 and compared it with footage taken the day of the attack.

Huong, 29, and Indonesian Siti Aisyah, 25, are being tried on suspicion of assassinating Kim, the estranged half-brother of North Korean leader Kim Jong Un, on Feb. 13 in the departure hall of the budget terminal.

The CCTV footage on Feb. 11 showed Huong picking out a stranger at random, approaching him from behind, and then appearing to swipe something on his face, according to Azirul.

"When the stranger turned toward her, Doan appeared to be saying 'sorry' by bowing her head and clasping her hands together. She then took two or three steps backward, turned around and walked away slowly," he told the court.

Azirul is expected to continue his testimony on Wednesday. The CCTV recordings will also be screened in court.

Huong and Aisyah are accused of killing Kim by smearing his face with VX, a highly lethal nerve agent that is listed as a weapon of mass destruction by the United Nations and is banned by the 1993 Chemical Weapons Convention.

(Royal Malaysia Police)

A postmortem report concluded that Kim died of VX nerve agent poisoning. CCTV footage from the terminal's departure hall showed the two women coming from behind, one after the other, and appearing to smear his face with a substance while he was standing near the check-in kiosk.

Last week, a government chemist testified that VX and its related products were found on the two women's clothing and nail clippings.

Continuing his testimony on Tuesday, Raja Subramaniam, who heads the Center for Chemical Weapons Analysis in the Chemistry Department of the Ministry of Science, Technology and Innovation, said the precursors and degradation products found on the two accused were non-toxic.

He repeated that washing under running water and scrubbing within 15 minutes after the skin comes into contact with VX could result in no effect or a delayed effect of the deadly poison.

Raja said he found the highest concentration of VX on Kim Jong Nam's blazer. VX was also found on Kim's face, eyes, T-shirt and plasma.

Earlier in the day, during cross examination by Aisyah's lawyer Gooi Soon Seng, Raja said he also analyzed "liquid" and gloves taken from Ri Jong Chol, a North Korean with a degree in chemistry who was briefly detained by the police before being released and deported in early March after they did not find enough evidence to charge him.

Gooi told reporters later that the items seized from Ri's apartment tested negative for VX.

After his release, Ri slammed the Malaysian authorities for allegedly using coercion to extract a confession from him. He denied he was at the airport or that his car was used. He claimed it was all a conspiracy to damage Pyongyang's reputation.

Ri was one of eight North Koreans whom police had identified previously with links to the murder.

South Korea and the United States have accused North Korean agents of masterminding the assassination.

Huong and Aisyah are charged with murder together with four North Koreans who fled the country just hours after the assassination.

Meanwhile, three other North Koreans were allowed to leave the country after police took their statements.

The women's defense is that they were duped into thinking the acts were just "pranks" for a hidden camera television show.