Sacramento, Calif. — It's an unusual sight to see: a Russian surveillance jet on the tarmac of Travis Air Force Base near Fairfield, California. It was first spotted last week and has been flying over some of the nation's most sensitive military sites — places where even U.S. commercial jets are banned, CBS Sacramento reports.

So why is a Russian spy plane flying over California?

There is a peaceful purpose to the missions and they began in Northern California. The aircraft is flown as part of the Treaty on Open Skies between the U.S. and Russian Federation, which took effect in 1992. Under the agreement, unarmed military planes from both countries are allowed to fly over each other's territory.

Russian spy plane Travis Air Force Base via CBS Sacramento

The Tupolev 154 is equipped with sensitive electro-optical cameras capable of taking high-resolution photos while airborne. The jet was flying with permission from the White House, according to officials.

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Data from the plane's transponder shows it flew over downtown Sacramento on its way to Travis last week. It also flew over other strategic military sites on the West Coast, including Point Mugu, Coronado Island, Camp Pendleton, 29 Palms, Vandenberg and Edwards Air Force Base. It even took a trip north of Las Vegas where the Nevada test site and Area 51 are located.

The flights were part of Russia's first mission over the U.S. this year.

An Air Force spokesperson said the plane was inspected for compliance before it took off from Travis. U.S. observers were also onboard the aircraft to monitor all phases of the flights.

The Air Force has its own version of an Open Skies surveillance plane, and so far this year, it's conducted three missions over Russian sites. In December, the U.S. carried out an "extraordinary" flight over Ukraine after Russia seized three Ukrainian naval ships off Crimea. The Pentagon, citing the Open Skies treaty, had said the U.S. and allies conducted the flight "to reaffirm U.S. commitment to Ukraine and other partner nations.‎"

The agreement is an effort by both countries to keep an eye on each other's military assets as new controversies revive some Cold War-era tensions.

There are 32 other nations that are also members of the Open Skies treaty. The Russian plane took off from Travis AFB on Saturday, which was also the day of the base's annual airshow.