Malaysia has warned that an investigation into the murder of the North Korean leader's half brother "may take longer than what we hope," as Pyongyang ally China said on Wednesday that no international action should be considered until it is finished.

Malaysia has said assassins used VX nerve agent, a chemical listed by the United Nations as a weapon of mass destruction, to kill Kim Jong Un's estranged half-brother, Kim Jong Nam, at Kuala Lumpur International Airport on Feb. 13.

"Due to the complexity and sensitivity of the case, investigation may take longer than what we hope for," Ahmad Nazri Yusof, Malaysia's permanent representative to the Organisation for the Prohibition of Chemical Weapons (OPCW), told a meeting of The Hague-based body on Tuesday.

"The government of Malaysia will fully cooperate with the OPCW and other international organizations to bring the perpetrators to justice," said Yusof, according to his statement posted on the OPCW website.

Under the Chemical Weapons Convention, OPCW states parties can "in cases of particular gravity" bring an issue to the attention of the U.N. Security Council and the U.N. General Assembly for possible action.

When asked if any action should be taken over the murder, China's U.N. Ambassador Liu Jieyi said on Wednesday: "The investigation is still going on, I think we need to see how the process will lead and what the true situation is."

The U.N. Security Council met behind closed doors on Wednesday to discuss North Korea's launch on Monday of four ballistic missiles.

British U.N. Ambassador Matthew Rycroft, president of the 15-member council for March, said the accusation of the use of VX nerve agent was raised by some council members.

"It came up, but there was not a particular proposal for the Security Council to take any action at this stage," he said.

U.S. officials and South Korean intelligence suspect North Korean agents were behind the assassination of Kim Jong Nam, who had been living in Macau under China's protection. He had spoken out publicly against his family's dynastic rule of North Korea.

"We absolutely see no place for chemical weapons in any situation whatsoever, so it's incredibly disturbing," U.S. Ambassador to the United Nations Nikki Haley told reporters after the Security Council meeting.

"It's one more factor for us to consider and one more factor that we know is an issue as we're dealing with how to possibly move forward with North Korea," she said.