WASHINGTON, March 6 (Xinhua) -- The U.S. State Department announced Tuesday it has imposed sanctions on the Democratic People's Republic of Korea (DPRK) over its alleged killing of top leader Kim Jong Un's half-brother with what authorities said was a nerve agent last year in Malaysia.

The United States determined on Feb. 22 that Pyongyang "used the chemical warfare agent VX to assassinate Kim Jong Nam, in the Kuala Lumpur airport," said Heather Nauert, the U.S. State Department spokesperson, in a statement.

Nauert said the determination was made under the U.S. Chemical and Biological Weapons Control and Warfare Elimination Act of 1991.

Effective since Monday after the finding was formally published, the sanctions build on existing U.S. sanctions to curb the DPRK's missile and nuclear weapons programs and restrict U.S. foreign aid and financial and military assistance, according to media reports.

That aid and assistance by the United States, however, never went to the DPRK, so the sanctions, which came on the same day when Kim Jong Un said was willing to talk to Washington and engage in denuclearization, were largely seen as symbolic.

A DPRK man was killed in the Kuala Lumpur airport, Malaysia, on Feb. 13, 2017. Malaysian police later confirmed that the victim, who was killed by VX nerve agent, a lethal chemical weapon, was Kim Jong Nam.