North Korea barred all Malaysian citizens from leaving the country, prompting Malaysia to bar North Koreans from leaving its nation as a tense rift escalated following the assassination of Kim Jong Un’s half-brother last month.

The move came just hours after North Korea’s ambassador to Malaysia was expelled from the country after he challenged an investigation into the death of Kim Jong Nam, who was allegedly poisoned by two women at Malaysia’s Kuala Lumpur airport in February. The ambassador, Kang Chol, reportedly angered Malaysian officials after saying North Korea “cannot trust” the investigation, which he earlier said had a “sinister purpose,” according to The New York Times.

Malaysian Prime Minister Najib Razak condemned the move as an “abhorrent act” and said his country would institute its own ban on North Korean citizens leaving Malaysia.

”As a peace-loving nation, Malaysia is committed to maintaining friendly relations with all countries,” Razak said in a statement posted to Facebook. “However, protecting our citizens is my first priority, and we will not hesitate to take all measures necessary when they are threatened.”

Kang has said Kim died of a heart attack.

Following Kang’s explusion, the Malaysian ambassador to Pyongyang was declared a “persona non grata” and expelled from North Korea, South Korea’s Yonhap News Agency said. He had already been recalled last month. The South China Morning Post reports there are currently 11 Malaysians in North Korea; nine at the Malaysian embassy and two working with the United Nations’ World Food Programme. It’s estimated 1,000 North Koreans work in Malaysian, according to The Associated Press.

North Korea’s state-run Korean Central News Agency said the temporary ban prohibiting the departure of Malaysians would stay in place “until the incident that happened in [the country] is properly solved.” KCNA said the ban was put in place to protect the North Korean citizens and diplomats in Malaysia, Reuters reported.

North Korea has drawn international rebuke since it fired four missiles into the Sea of Japan late Sunday evening, three of which fell in Japan’s exclusive economic zone. The test launches, personally overseen by Kim Jong Un, were conducted in retaliation over the start of a massive joint military exercise by the U.S. and South Korea, dubbed Foal Eagle.

“In the hearts of artillerymen ... there was burning desire to mercilessly retaliate against the warmongers going ahead with their joint war exercises,” KCNA said.

The launches were routinely slammed by international leaders. A spokesman for the State Department said America was “prepared to use the full range of capabilities at our disposal against this growing threat.”

U.S. Pacific Command began deploying the first parts of an advanced antimissile system, dubbed THAAD, in South Korea on Monday, less than a day after the launch.