Stephen Biegun, the U.S. envoy to North Korea, urged Pyongyang on Monday to restart nuclear negotiations with Washington, saying "Let's get this done."

"It is time for us to do our jobs. Let's get this done. We are here and you know how to reach us," Biegun said at a news conference in South Korea.

North Korea had set a deadline for a nuclear deal for the end of the year, where Kim Jong Un's regime would commit to a denuclearization program in exchange for reduced sanctions. North Korea has threatened the U.S. with a "Christmas gift" if there continues to be no progress towards sanctions relief.

Biegun has called North Korea's threats towards the U.S. "hostile, negative and so unnecessary."

The last time U.S. and North Korea negotiators met was in early October in Sweden, with nuclear talks falling through on the first day of discussions.

President Trump and Kim have met each other several times to discuss a nuclear deal, convening in Vietnam in February and Singapore in June 2018 for negotiations. The two leaders were unable to reach an agreement.

Biegun's comments come after Trump seemingly issued a warning to Kim after North Korea conducted a long-range missile test.

"Kim Jong Un is too smart and has far too much to lose, everything actually, if he acts in a hostile way," read Trump's tweet on Dec. 8. "He does not want to void his special relationship with the President of the United States or interfere with the U.S. Presidential Election in November."

Trump has considered North Korea's Kim as a friend but Washington and Pyongyang have long had a strained relationship. In September 2017, Trump called Kim "Rocket Man" in front of the United Nations General Assembly, with Kim saying Trump is "mentally deranged" in response.

Under former President Barack Obama, the U.S. had expanded sanctions towards Pyongyang as the country advanced its nuclear program. President George W. Bush had declared North Korea as part of an "axis of evil" along with Iran and Iraq under former dictator Saddam Hussein.