"In reference to the representation submitted on March 11 to the honourable attorney-general, we got an order to proceed with the case", lead prosecutor Muhammad Iskandar Ahmad told the High Court in Shah Alam, outside Kuala Lumpur.
Vietnam's ministers of justice and foreign affairs are communicating with their Malaysian counterparts to secure his client's release, Teh added.
"God knows that Siti and I did nothing wrong", she said.
In a statement, Bukit Gelugor MP Ramkarpal Singh said subjecting Doan to further prosecution, but not her co-accused, "particularly when a prima facie case has been found against both of them is, with respect, unprecedented and regrettable".
"Very obviously, there is discrimination".
Huong and Aisyah were accused of smearing the toxic VX nerve agent on Kim Jong Nam's face in an airport terminal in Kuala Lumpur on February 13, 2017.
They were the only people in custody after four North Korean suspects fled the country the same morning Kim was killed.More news: Man uses mobile phone as shield in alleged bow and arrow attack
Doan and Siti Aisyah were initially charged with the murder of Kim Jong Nam, the half-brother of North Korean leader Kim Jong Un.
Prosecutors said Aisyah and Huong were well-trained assassins but their lawyers argued the four North Koreans were the masterminds.
The court ordered Indonesian Siti Aisyah and Doan to enter their defence on the murder charge on August 16, 2018.
Huong's lawyer sought a deferment of the trial Thursday, saying she was unwell and needed medical treatment.
"I have no idea what is going on", a exhausted and pale-looking Huong, who was wrapped in a red headscarf and a coat, said through an interpreter.
The presiding judge said the defendant was not "physically and mentally" well enough to continue with the proceedings. The defense phase of the trial was to have begun Monday.
A Malaysian court on Thursday announced it would temporarily postpone the trial of Vietnamese citizen Đoàn Thị Hương, a suspect in the murder of a North Korean national in 2017, due to her health condition, Vietnam News Agency reports. I'm very sad and disappointed. She is now back in Indonesia.More news: Disney's 'Aladdin': Mena Massoud & Naomi Scott Featured In New Poster
He said he believes his daughter is innocent and told her to "stay calm and hope for the best outcome".
The ruling leaves Ms Doan, who arrived with armed guards at the Shah Alam court shortly before 9am and wearing a bulletproof vest, as the sole suspect in the case still behind bars.
"It's so unfair. They were together, did the same thing", she said.
Lawyers for Huong - who could face death by hanging if convicted - then asked the Malaysian government to withdraw the murder charge against her.
"So it makes no difference whatsover if Doan's image was caught on the CCTV camera, none at all", he said after the hearing. He urged the attorney-general to be transparent and explain why he dropped the case against Aisyah but not Huong. Kim died in 2017 after a toxic nerve agent was smeared on his face at Kuala Lumpur Airport.
Both women maintain they were tricked into carrying out the attack, which they thought was part of a TV reality show. There was security camera footage of Huong.More news: Sony reportedly looking to buy Rockstar publisher Take-Two Interactive