A Japanese broadcaster reports that a U.S. reconnaissance aircraft left its base in Japan and flew over the East Sea amid speculation that North Korea may launch an intercontinental ballistic missile(ICBM) as early as week.

NHK said the RC-135S Cobra Ball, which is capable of collecting data from launched ballistic missiles, left Kadena Air Base in Okinawa Prefecture around 2:30 a.m. Tuesday and returned nine hours later at 11:40 a.m.

NHK captured footage of the aircraft returning to base. Citing the flight-tracking service Flightradar24, the report said the spy plane appears to have flown back and forth over the sea east of the Korean Peninsula.

The Cobra Ball, only three units of which exist in the world, can track the trajectory of a ballistic missile from a long distance using state-of-the-art electro-optical equipment.

The flight comes amid reports that Pyongyang could launch an ICBM following tests in recent days which U.S. and South Korean intelligence assessed to be ICBM launches.

An official at South Korea's Joint Chiefs of Staff told reporters Monday that it's difficult to predict the exact time but intelligence authorities in Seoul and Washington are closely monitoring the regime's moves and maintaining a firm defense posture.