In what military watchers regard as an unusual move, the U.S. military sent four surveillance planes, simultaneously, over and around the Korean Peninsula this week in reaction to North Korea's cryptic reference to a possible "Christmas gift" for the U.S.

The action followed reports last week that North Korea had warned of a possible missile launch or nuclear test for late December amid stalled nuclear negotiations with the U.S.

The U.S. aircraft were detected by Aircraft Spots, an aviation tracking site, according to a report by South Korea's Yonhap News Agency.

The site claims that four U.S. planes -- identified as RC-135W Rivet Joint, E-8C, RQ-4 Global Hawk and RC-135S Cobra Ball -- flew their missions between Christmas Eve and early Christmas Day, the report said.

The first two planes flew at 31,000 feet while the Global Hawk flew at 53,000 feet, the tracking site said. Meanwhile, the Cobra Ball and a refueling plane flew over the East Sea.

Earlier this month, Ri Thae Song, a North Korean vice foreign minister for U.S. affairs, expressed doubts that the U.S. was negotiating in good faith, accusing the Americans instead of stalling as it tried to get better terms.

Related: Trump Says North Korea May Be Planning Nice 'Christmas Gift'

"The dialogue touted by the U.S. is, in essence, nothing but a foolish trick hatched to keep the DPRK bound to dialogue and use it in favor of the political situation and election in the U.S.," Ri said. "What is left to be done now is the U.S. option -- and it is entirely up to the U.S. what Christmas gift it will select to get."

President Trump has downplayed the matter, despite urging from some analysts and advisers in Washington that he take a harder line against Pyongyang.

When asked Tuesday in Florida whether he was concerned about a long-range missile test, Trump quipped that perhaps North Korean dictator Kim Jong Un was planning a nice gift.

"We'll see what happens. Let's see. Maybe it's a nice present, maybe it's a present where he sends me a beautiful vase as opposed to a missile test ... you never know," the president said.

But Trump stressed that the U.S. was prepared to deal with any kind of "gift" the North might send.

"We'll find out what the surprise is, and we'll deal with it very successfully," Trump told reporters at his Mar-a-Lago resort in Florida, according to Yonhap. "Everybody's got surprises for me, but let's see what happens. I handle them as they come along."

--The Associated Press contributed to this story.