U.S. F-22 stealth fighter aircraft scrambled on Saturday to intercept four Russian reconnaissance jets off Alaska, said NORAD, the U.S and Canadian defense organization.

The interception of the Russian Tu-142s marks the 10th time this year that Russian military aircraft have been intercepted off Alaska, the North American Aerospace Defense Command, or NORAD, said in a statement.

The Russian aircraft did not enter U.S. or Canadian sovereign airspace, NORAD said.

The intercepting aircraft are based at Elmendorf Air Force Base in Alaska, said a NORAD spokesman, Captain Cameron Hillier.

The incident is one of a series of back-and-forth probes by Russia and the United States this year. On June 19, Russia scrambled fighter jets to intercept two U.S. B-52 bombers flying over the Sea of Okhotsk, off Russia’s far eastern coast, the Interfax news agency reported.

Reuters