President-elect Donald Trump issued a warning to North Korea on Monday, saying its plans to finish building a nuclear weapon capable of striking the United States “won’t happen!”

“North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S. It won’t happen!” Trump wrote on Twitter Monday evening.

On Sunday, North Korean leader Kim Jong Un said in a New Year’s Day address that preparations for launching an intercontinental ballistic missile have “reached the final stage.”

Trump shot back at the claim, while also accusing China of not intervening more on behalf of the United States.

“China has been taking out massive amounts of money & wealth from the U.S. in totally one-sided trade, but won’t help with North Korea. Nice!” Trump tweeted.

Earlier in Monday, Trump took aim at Chicago Mayor Rahm Emanuel, saying that if Emanuel couldn’t curb growing murder rates in the nation’s third largest city, he should appeal to federal authorities.

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“Chicago murder rate is record setting — 4,331 shooting victims with 762 murders in 2016,” Trump tweeted, citing figures released Sunday by the Chicago Police Department. “If Mayor can’t do it he must ask for Federal help!”

Chicago police data show the city ended 2016 with its largest number of murders in 20 years, while also experiencing an uptick in shootings. There were 1,100 more shooting incidents last year than in 2015, the data show.

Emanuel’s spokesman, Adam Collins, said in a statement to reporters: “We are heartened he [Trump] is taking this issue seriously and look forward to working with the new administration on these important efforts.”

“As the president-elect knows from his conversation with the mayor, we agree the federal government has a strong role to play in public safety by funding summer jobs and prevention programming for at-risk youth, by holding the criminals who break our gun laws accountable for their crimes, by passing meaningful gun laws and by building on the partnerships our police have with federal law enforcement,” Collins said.

Emanuel, who was President Barack Obama’s White House chief of staff before leaving in 2010 to run for mayor of Chicago, met with the president-elect behind closed doors at Trump Tower on Dec. 7, but the bulk of their discussion focused on immigration issues, Emanuel told reporters after the meeting.

Asked whether Trump brought up Chicago’s crime issues, Emanuel responded curtly at the time: “We talked about public safety.”

The president-elect returned to his namesake Manhattan tower late Sunday night after spending two weeks at his Palm Beach, Florida, resort. Also Monday, he resumed meeting with his top advisers at Trump Tower, including an evening session with Vice President-elect Mike Pence.