The United States is closely monitoring activity from North Korea over fears that it might test launch a long-range missile this Christmas. Experts noted that activity in one of the communist country’s missile testing sites picked up this past month.

Satellite photos show that activity ramped up at a North Korean worksite that is suspected to be a production facility for the nation’s long-range nuclear missiles, Daily Mail reported. As a result, there are now speculations that Pyongyang might be on the verge carrying an ICBM (intercontinental ballistic missile) test soon.

The photos show a structure on the site which has just been built this month. Some experts believe that the structure is temporary and is needed to raise the missile’s launcher arm.

“We believe North Korea erects this structure when the facility is involved in producing or modifying ICBM launchers,” East Asia Nonproliferation Program director Jeffrey Lewis said.

North Korean leader Kim Jong-un recently convened a meeting of the nation’s key ruling party figures on Sunday, which further raised alarms that Pyongyang might choose to set aside diplomatic efforts. The meeting addressed “important organizational and political measures and military steps to bolster up the overall armed forces,” as reported by the Korean Central News Agency (KCNA).

On the same day, President Donald Trump discussed the situation over the phone with his Japanese counterpart Shinzo Abe. During the phone call that lasted 75 minutes, the two leaders discussed on a “coordinated response” against North Korea.

“We [he and Trump] analyzed the latest developments regarding North Korea and had a thorough discussion about the steps ahead,” Abe told Nikkei Asia Review.

North Korea has been increasingly combative in its statements recently due to the U.S.’ refusal to lift economic sanctions. International concern is mounting due to Pyongyang hinting of a “Christmas surprise” for the U.S. early this month.

“North Korea has been advancing,” former State Department official Anthony Weir said. “It has been building new capabilities. As long as that continues, they gain new capabilities to try new missiles to threaten us and our allies in new ways.”