Pentagon sources tell CBN News, U.S. military officials in Seoul, South Korea have detected "some movement from the North's missile facilities and bases" and maintained "the heightened reconnaissance and preparedness posture."
The United States has reportedly stepped up flights by its U-2S ultra-high altitude reconnaissance aircraft.
The warning comes after a top Russian politician predicted Pyongyang was on the verge of testing a missile capable of reaching the United States mainland.
After visiting the North Korean capital, Anton Morozov told reporters the regime "intend to launch one more long-range missile in the near future."
He added that "their mood is rather belligerent", in comments made to RIA news agency.
October 10, marks the anniversary of the founding of the ruling Worker's Party of Korea.
The North has been known to celebrate the event with fireworks.
Meanwhile, Russia and China are calling for restraint on North Korea after U.S. President Donald Trump warned over the weekend "only one thing will work" in dealing with Pyongyang, hinting military action was on his mind.