The unexpected release of an Indonesian woman accused of killing the estranged half-brother of North Korea’s leader has spurred Vietnam to demand her co-defendant be set free as well.

Foreign Minister Pham Binh Minh called his Malaysian counterpart, Saifuddin Abdullah, on Tuesday and pleaded Doan Thi Huong be given a fair trial and set free, according to a statement posted on the ministry's website.

The call from Vietnam’s foreign minister was an unprecedented public request from Hanoi, which generally does not get involved in criminal cases involving its citizens overseas.

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Huong, whose murder trial is expected to continue Thursday, and her co-defendant, Indonesian Siti Aisyah, were accused of killing Kim Jong Nam by smearing VX nerve agent on his face while at a Malaysian airport in 2017.

Vietnam is urging Malaysia to release Doan Thi Huong, the second woman accused of killing the estranged half brother of North Korea’s leader. (AP)

Both women, who were accused along with four missing North Koreans, have said they thought they were playing a prank for a TV show.

On Monday, prosecutors in Malaysia unexpectedly dropped the murder charge against Aisyah, who has since returned to her home village. They did not give any reason for the remarkable retreat in their case against her.

"[I was] shocked at first, I couldn't believe it," Aisyah said of her release. "But I'm happy."

Indonesian Siti Aisyah, left, smiles next to her lawyer Gooi Soon Seng after a press conference at Indonesian Embassy in Kuala Lumpur, Malaysia, Monday, March 11, 2019. (AP)

Aisyah said she hopes Huong is also freed.

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Prosecutors are expected on Thursday to reply to a request by Huong’s lawyers for the government to withdraw the murder charge against her as well.

Indonesia's government said its continued high-level lobbying had resulted in Aisyah's release and alleged the young migrant worker had no idea she was being "manipulated by North Korean intelligence."

Meanwhile, Vietnam had pushed less hard on behalf of Huong, even recently hosting North Korean leader Kim Jong Un for an official visit and a summit with President Donald Trump. The call between the foreign ministers came as a surprise as Vietnam had refrained from publicly lobbying for her release until Tuesday.

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Huong's lawyer, Hisyam Teh Poh Teik, said Huong felt Aisyah's discharge was unfair to her because the judge last year had found sufficient evidence to continue the murder trial against them.

"She is entitled to the same kind of consideration as Aisyah," he said. "We are making representation to the attorney general for Doan to be taken equally...there must be justice."

Malaysian officials have never officially accused North Korea and have made it clear they don't want the trial politicized.

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Kim was the eldest son in the current generation of North Korea's ruling family. He had been living abroad for years but could have been seen as a threat to Kim Jong Un's rule.

The Associated Press contributed to this report.