The US "strongly condemns the use of chemical weapons to conduct an assassination", State Department spokeswoman Heather Nauert said in a statement.
It also pledged not to use nuclear or conventional weapons against the South, he said.
The investigation found Kim Jong-nam was murdered with the highly toxic liquid nerve agent VX used as a chemical weapon, resulting in automatic sanctions in compliance with the Chemical and Biological Weapons Control and Warfare Elimination Act of 1991.
The North Korean leader assumed power after his father, Jong-il, died in late 2011. The statement offered no further details on how the suspected link between the chemical agent and the North Korean government was definitively proven.
But in a somewhat encouraging sign, North Korea did agree to freeze any missile tests while talks are going on.
Washington has been leading diplomatic efforts to ramp up international pressure on North Korea.
In 2003, China-led six-party talks, bringing together the United States, Russia, Japan, China and both Koreas in a torturous process.
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And for the first time, South Korea reports that its neighbor is willing to consider dismantling its nuclear program if the US moves first, and eliminates any military threats to the North Korean regime.
Curbs have also been toughened on the export of industrial machinery, transportation vehicles, iron, steel and other metals. "It would be a great thing for the world, it would be a great thing for North Korea, it would be a great thing for the peninsula".
Pyongyang had expressed their willingness to talk, but the US had remained rather skeptical of the North's intentions in improving its ties with the South, saying US-North Korea dialogue would only be held if North talks about denuclearization.
Mr Kim and South Korean President Moon Jae-in will meet on the heavily fortified border next month, at the truce village of Panmunjom. "It seems to us that the conditions have been met for North Korea-U.S. talks".
He added: "May be false hope, but the US is ready to go hard in either direction!". To achieve this, the two parties agreed to hold working-level consultations. Previously, he said, those were not seen as adequate by North Korea.
But opposition leaders expressed skepticism over Moon's claim that North Korea made clear its willingness to denuclearize. "We don't aim for that to happen and it's also impossible".
Löfven described the situation on the Korean Peninsula said one of the world's "key regional and global security challenges".
The US has announced new, unspecified sanctions against Pyongyang for using a chemical weapon to assassinate Kim Jong-nam, the half-brother of North Korean leader Kim Jong-Un, in February 2017.