The U.S. military has confirmed that four Russian military aircraft were tracked operating near the U.S. state of Alaska late Sunday and early Monday.

North American Aerospace Defense Command (NORAD) said that it had “detected and tracked” the aircraft operating in the Alaska Air Defense Identification Zone (ADIZ).

“The Russian aircraft remained in international airspace and did not enter American or Canadian sovereign airspace. This Russian activity in the Alaska ADIZ occurs regularly and is not seen as a threat,” NORAD said in a statement Monday.

The ADIZ is international airspace directly next to sovereign airspace “that requires the ready identification, location and control of all aircraft in the interest of national security,” according to NORAD.

“NORAD remains ready to employ a number of response options in defense of North America,” the command said.

The U.S. military has confirmed multiple Russian military aircraft operations in the ADIZ this year.

In May, U.S. F-16 and F-22 fighter jets intercepted six Russian aircraft, including TU-95 bombers, SU-35 fighter jets and IL-78 tanker aircraft that had entered the Alaska ADIZ. Less than a week later, NORAD tracked another Russian military aircraft operating in the ADIZ.

NORAD said earlier that these Russian aircraft did not enter U.S. or Canadian sovereign airspace.

The May flights occurred as several planned large-scale military training exercises were underway in and around Alaska.