South Korean media reported on Wednesday that the United States flew four surveillance aircraft over the Korean Peninsula simultaneously to keep an eye out for North Korea’s promised “Christmas gift” of a provocative missile launch.

“It is unusual for four American surveillance planes to conduct missions around the Korean Peninsula at the same time. That appears to illustrate how much attention the U.S. is paying to an increasingly belligerent North Korea,” South Korea’s Yonhap News wrote.

The report was based on information from a civilian air tracking group called Aircraft Spots, which monitored three manned aircraft – an RC-135W Rivet Joint, an E-8C Joint STARS, and an RC-135S Cobra Ball – as well as an RQ-4 Global Hawk drone. A KC-135R refueling aircraft was also deployed to support the planes. The RC-135S launched from Kadena Air Base in Japan and covered the Sea of Japan, commonly known as the East Sea in Korea.

Yonhap News also noted that land and naval radar systems were on alert throughout Christmas Day. As of Thursday morning, no missile launch had been detected.

NBC News noted a high level of U.S. surveillance activity on the Korean Peninsula this week. While the Pentagon did not respond to NBC’s request for comment, a South Korean defense spokesman said his government and the United States are working closely on “monitoring and tracking down North Korean movements.”

In early December, North Korean Vice Foreign Minister Ri Thae-song denounced American diplomacy as a “foolish trick” and declared Pyongyang was no longer interested in negotiations.

“The DPRK [North Korean regime] has done its utmost with maximum perseverance not to backtrack from the important steps it has taken on its own initiative. 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,” he warned.

This was widely interpreted as a threat to conduct missile tests on or before Christmas Day, both to continue North Korea’s policy of ratcheting up tensions to force concessions and because such activity could prove embarrassing to U.S. President Donald Trump during his reelection campaign.

A week after Ri’s remarks, North Korea announced it had conducted a “very important test” of rocket engines that could be used in intercontinental ballistic missiles. Commercial satellite photos later detected activity at a factory where North Korea has constructed long-range missiles and launchers.

Speaking with reporters on Christmas Eve, President Trump joked about Ri’s threat: “Maybe it’s a present where he sends me a beautiful vase as opposed to a missile test, right? I may get a vase, I may get a nice present from him. You don’t know. You never know.”