U.S. officials have quietly stepped up security measures on inbound flights from Russia after two commercial jets crashed within minutes of each other after departing from a Moscow airport earlier this week.

The move reflects U.S. authorities' concern that the crashes, which killed 89 people, may have been acts of terrorism, even though Russian officials continue to suggest they were coincidental accidents.

Los Angeles residents reported seeing U.S. fighter jets escort an Aeroflot Russian Airlines flight Wednesday on an unusual path over the ocean on approach to the airport. An official familiar with the security procedures said military jets are likely to continue tracking Russian flights into U.S. cities, including those bound for Washington, New York, San Francisco and Seattle.

U.S. security officials said they did not request the escorts but the military unit in charge of domestic defense, the North American Aerospace Defense Command, took the action on its own. The officials said they are considering whether to require carriers operating flights to the United States from Moscow -- Delta Air Lines and Aeroflot -- to tighten security on those routes. Such measures could include additional screening of passengers and their luggage before boarding, extra screening of luggage and the placement of armed air marshals on board.

"We constantly evaluate security measures in the United States every day," said Brian Roehrkasse, spokesman for the Department of Homeland Security. "Based on our current assessment of intelligence and threat reports, we are not implementing any additional security measures at this time."

A NORAD spokesman said the military unit does not discuss its operations.

U.S. officials declined to comment publicly as to whether the plane crashes in Russia -- neither of which involved Russian carrier Aeroflot -- appeared to be accidental, saying that it was up to Russian investigators to determine the causes of each crash. U.S. officials offered to provide technical assistance to the investigations but the Russian government has not yet requested it, a State Department spokesman said.

Even though intelligence reports do not indicate any new terrorist threats to U.S. airliners, security officials said they are monitoring the events in Russia because they come at a time when security is already being tightened this weekend in New York for the Republican National Convention.

"We are closely monitoring the crash investigations," said one U.S. security official.