The United States flew four surveillance planes over the Korean peninsula Wednesday, according to an aviation tracker, South Korea's Yonhap news agency reported.

The move comes after North Korea threatened to give Washington a “Christmas gift” unless the U.S. makes concessions in nuclear talks with the country. Negotiations between U.S. and North Korean officials stalled earlier this year after a summit between President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE and North Korean leader Kim Jong Un Kim Jong UnFormer GOP lawmakers on endorsing Biden: Trump is no Republican, 'lacks basic self-control' North Korean leader Kim apologizes over killing of South Korean official Pelosi knocks Trump over refusal to commit to peaceful transfer of power MORE ended abruptly.

The U.S. flew four aircraft, the RC-135W Rivet Joint, E-8C, RQ-4 Global Hawk and RC-135S Cobra Ball, over the country between Tuesday and early Wednesday. The planes were believed to have carried out missions in and around the Korean Peninsula, Yonhap reported.

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The news agency called the move unusual and said it shows the U.S.’s response to North Korea’s threat. North Korea also reportedly carried out two rocket engine tests at a satellite site, later saying the results will have “an important impact” on the country’s position.

Military sources confirmed to the news outlet that South Korea and the U.S. have strengthened efforts against the possibility of North Korea firing an ICBM or other weapons.

"We're keeping a close watch over military moves in North Korea," said one of the sources, who the outlet declined to name.

Trump has said the U.S. will “deal with” whatever North Korea has planned behind its threat, joking that the North Korean leader could send him a “nice present” instead of planning a missile test.

“Let’s see what happens. Everybody’s got surprises for me, but let’s see what happens. I handle them as they come along,” Trump told reporters Tuesday in Palm Beach, Fla.

“We’ll see what happens,” Trump continued. “Maybe it’s a nice present. maybe it’s a present where he sends me a beautiful vase as opposed to a missile test.”