In late October when two Russian Tupolev bombers approached the aircraft carrier USS Ronald Reagan in the Sea of Japan, the U.S. sent up fighters to intercept.

In May, fighters were dispatched to intercept Russian bombers and fighters off Alaska.

But last month a Russian Tu-154 jet conducted a surveillance flight unchallenged over military installations in Hawaii, U.S. government officials confirmed.

The Russian overflight conducted Oct. 4-6 was sanctioned by the United States as part of the Treaty on Open Skies, which has been in effect since 2002 to verify arms control agreements and promote stability through transparency of military forces and activities.

Russian planes fly over U.S. military installations capturing imagery, and the same is true for U.S. planes and Russia. Some 34 countries are part of the overflight deal.

The flights are not publicized and many people are not aware they even occur. The Russians did cause something of a stir with an overflight of Washington, D.C., as low as 4,000 feet in August.

But Moscow soon will be limited to about half the recent overflight distance over Hawaii per a new restriction by the United States. The curb is a probable tit for tat over worsening relations and what an Open Skies observer called a “poke in the eye” to Russia for limiting the flying time over Kaliningrad on the Baltic Sea to 310 miles on each mission.

The Tu-154M, a three-engine jetliner from the 1960s, even landed here for refueling, according to Pacific Air Forces and the Defense Threat Reduction Agency.

“The observation flight consisted of Hawaii, Maui, Molokai, Oahu and Kauai covering 894 nautical miles (1,029 statute miles) on the Tu-154M,” Pacific Air Forces said in an email. “The aircraft landed at Joint Base Pearl Harbor-Hickam for refueling, servicing and crew rest.”

Asked about other overflights in recent fiscal years, Pacific Air Forces said, “The Russian Federation has conducted two overflights of the Hawaiian Islands during 2016 and 2017.” Before that, Hawaii overflights occurred less frequently, according to one observer.

But at the Open Skies Consultative Commission in Vienna on Sept. 26, the United States announced it would decrease the permissible flight distance over the Hawaiian Islands, cease waiving certain Federal Aviation Administration restrictions, and discontinue allowing Russia overnight accommodations at some Open Skies refueling airfields, Pacific Air Forces said.

The Wall Street Journal reported the United States will limit the Russian overflights in Hawaii to 900 kilometers, or 560 miles. That means about half the flying distance over Hawaii as in October. The change is expected to come into force Jan. 1, but the Defense Threat Reduction Agency would not confirm the date.

Steffan Watkins, a Canada-based information technology security professional who researches open-source intelligence and is well versed on the Open Skies treaty, said the Hawaii restrictions are meant to cause maximum inconvenience for Russia.

The Wall Street Journal said the actions are a protest against Russian Open Skies restrictions over Kaliningrad, where advanced air defenses are based, but are also an indicator of the larger split in U.S.-Russia relations.

Watkins said the Tu-154M flies from Moscow to Iceland to Canada to California to Hawaii to make the overflights. The actual observation mission begins and ends at Daniel K. Inouye International Airport.

Once Russia conducted the overflight, which he believes was on Oct. 5, the Tu-154M had to fly back to Moscow the same way it came. Cutting the Hawaii overflight miles means the Russians would have to make a return trip to match what they did in logging 1,029 miles over Hawaii military installations in one mission.

Getting to Hawaii “is by far the farthest that they could possibly fly for this mission,” Watkins said. “And now the U.S. is saying, ‘Oh, sorry, you are going to do it twice.’”

The occurrence of Russian overflights for what apparently has been three years in a row, including this year, likely indicates an increased interest in Hawaii, which “is a communications hub for all sorts of data,” Watkins said. “And I bet Russia wants to keep an eye on it now more than ever.”

The Hawaii restriction isn’t “because the U.S. is trying to hide something in Hawaii or that they are worried about something in Hawaii,” he said. “This is just maximum pain so they can call it a reciprocal thing because of the Kaliningrad restrictions.”

Watkins said the Tu-154M’s Mode-S transponder signal can be triangulated by four or more radio aviation hobbyists. Collected by the site FlightRadar24, the track showed the Russians were doing a “lawn mower” pattern over Oahu, “which is unusual, but would cause maximum disruption to the Honolulu international airport, which is coincidentally what Russia accuses the USA of doing to Kaliningrad — disrupt their airspace,” he said. “Was this a petty tit for tat? Was this just a thorough look at the island? I don’t know.”

The Open Skies treaty allows unarmed aerial observation flights over the entire territory of participants. Mutual aerial observation was first proposed by President Dwight D. Eisenhower in 1955 to reduce the risk of war, but the treaty itself became effective in 2002.

The U.S. State Department said Open Skies aircraft can be equipped with video, optical panoramic and framing cameras, infrared sensors and synthetic aperture radar. U.S. personnel fly on the Russian flights and vice versa to ensure treaty compliance.

In 2016, Russia became the first treaty member to install approved digital electro-optical sensors on its two Open Skies aircraft, while the United States and other countries still use older “wet film” cameras.

The State Department said the treaty “limits all optical sensors,” whether digital or “wet film,” to 30-centimeter resolution — a level that allows parties to distinguish between a tank and truck, and is similar to Google Earth. In any case, the United States gets better imagery from its constellation of observation satellites.

Film cameras are increasingly obsolete and the United States is “actively preparing for the transition to digital electro-optical sensors” for Open Skies, the department said. Some military brass and members of Congress have decried the Russian Open Skies digital sensors, however.

Lt. Gen. Vincent Stewart, then director of the Defense Intelligence Agency, was asked at a 2016 House Armed Services Committee hearing whether the digital technology could help the Russians target the United States.

“This Open Skies discussion is, think Polaroids in the 1960s, ’70s and ’80s vs. 1080 high-definition capability,” which allows Russia to get “incredible foundational intelligence,” Stewart said. He added he would “love to deny the Russians having that capability.”

Watkins said the State Department is pro-Open Skies as a diplomacy builder and the Defense Intelligence Agency is anti-Open Skies. An August Congressional Research Service report said that when the United States first signed Open Skies, “most analysts agreed that the treaty would provide little information not already available from observation satellites.”

The flights are seen as more useful to Russia. But the treaty also benefits participants who do not have observation satellites, leading to a more secure European continent, the research service said.