(CNN) The United States government says it's sanctioning North Korea after deeming its government responsible for the murder of Kim Jong Un's estranged half-brother Kim Jong Nam.

It's unclear why the US State Department chose to make the announcement Tuesday, hours after South Korea said the Kim regime would be willing to speak with the US about giving up its nuclear weapons.

Kim Jong Nam died in Kuala Lumpur last year after being exposed to VX, a nerve agent so deadly the United Nations considers it a weapon of mass destruction. Malaysian prosecutors allege that two women wiped Kim's face with the substance in broad daylight at an airport in the country's capital. He died minutes later in an ambulance, en route to the hospital.

The US State Department revealed Tuesday that new sanctions passed in response to Kim's killing had taken effect on March 5 . However, they're unlikely to have much practical effect as they appear to overlap with the litany of measures already levied against North Korea in response to its ballistic missile development and nuclear weapons proliferation.

US State Department spokeswoman Heather Nauert said in a statement that American authorities determined the Kim Jong Un regime was responsible for the killing on February 22, a day before the US Treasury Department announced a tranche of measures targeting Pyongyang's illicit shipping operations.

"The United States strongly condemns the use of chemical weapons to conduct an assassination," Nauert said. "This public display of contempt for universal norms against chemical weapons use further demonstrates the reckless nature of North Korea and underscores that we cannot afford to tolerate a North Korean WMD program of any kind."

The statement came after a day of extraordinary turn of events on the Korean Peninsula.

. After returning from an unprecedented trip to Pyongyang, South Korean National Security Chief Chung Eui-yong reported that Kim would be willing to engage in talks with the US

It's believed to be the first time Kim has ever met any South Korean government official since taking power in 2011.

The North Korean leader also took the unusual step of inviting his delegation to dine in the Korean Workers' Party headquarters Monday, sharing a table with his wife Ri Sol Ju and sister Kim Yo Jong.

After the talks, Kim agreed to refrain from conducting nuclear and missile tests while engaging in dialogue with South Korea, Chung said.

Two women on trial

Malaysian authorities revealed shortly after Kim Nam's death that they believed the North Korean regime was ultimately responsible, a charge Pyongyang strongly denies.

Two women, Indonesian national Siti Aisyah and Vietnamese national Doan Thi Huong, are on trial for murder and face the death penalty if convicted. Their lawyers maintain they were duped by North Korean agents into thinking they were wiping a harmless substance on Kim's face for a prank television show.

North Korea is believed to have one of the world's largest chemical weapons stockpiles and has faced allegations of selling both its arms and expertise abroad.

A portion of a leaked report from a United Nations panel charged with monitoring North Korea sanctions enforcement accused Pyongyang of supplying Syria with chemical weapons equipment and expertise.

The allegations surfaced just days after the regime of Syrian President Bashar al-Assad was accused of carrying out a chlorine gas attack in Eastern Ghouta, a suburb of the capital of Damascus.

The Assad regime has repeatedly denied claims it uses chemical weapons. North Korea has also denied the allegations in the UN report.