BANGKOK — The use of a banned chemical nerve agent in assassinating the half brother of North Korea’s ruler at a Malaysian airport last month was “totally unacceptable,” Prime Minister Najib Razak of Malaysia said in his first public comments on the case.

In a televised interview that aired on Sunday, Mr. Najib said the use of VX nerve agent could have killed many more people than just the intended victim, Kim Jong-nam, the estranged elder brother of the North Korean leader, Kim Jong-un.

“We have to accept the fact that a crime has been committed in Malaysia,” he said in the interview with Al Arabiya television. “The substance, or the weapon used, is a very, very dangerous chemical weapon, which should not be used at all, because if used in large quantities, many, many people could have been killed, not just one person.”

Mr. Kim was killed on Feb. 13 as he prepared to check in for a flight at Kuala Lumpur International Airport. The police said that two women, one from Vietnam and one from Indonesia, smeared the nerve agent on his face. They have been charged with murder.

Mr. Najib did not mention either Kim brother or North Korea by name. And he stopped short of accusing the North Korean government of staging the assassination, an assertion that has been made by the South Korean government. The Malaysian police are seeking seven North Korean men in the case, including a diplomat stationed in Malaysia.

On Saturday, Malaysia declared the North Korean ambassador, Kang Chol, “persona non grata” and ordered him to leave the country within 48 hours.

“We have to take a very serious stance,” Mr. Najib said. “We are very determined to find out the truth and that the people responsible should be brought to justice.”

The interview was recorded last week before the two women were charged with murder and before Malaysia ordered Mr. Kang’s expulsion.

“The use of chemical weapons in Malaysia is totally unacceptable,” the prime minister said.