Washington (CNN)Russian military aircraft were spotted flying off the coast of Alaska for the fourth time in as many days, a US defense official told CNN on Friday.

The two most recent sightings occurred late Wednesday and on Thursday, with the first involving two IL-38 maritime patrol aircraft and the second involving two Tu-95 nuclear-capable Bear bombers.

Russian aircraft never entered US airspace and it is unknown at this time whether the US Air Force scrambled any aircraft in response to either occurrence.

The bombers entered the Alaskan Air Defense Identification Zone 700 nautical miles southwest of Anchorage -- significantly farther from the US coastline than two other encounters that occurred on Monday and Tuesday.

While these flights pose no real military threat, US defense officials are taking notice of the high frequency at which they've occurred this week.

There is "no other way to interpret this other than as strategic messaging," the official told CNN.

Earlier in the week, US defense officials called recent sightings of the bombers "nothing out of the ordinary" -- itself an indication that both nations are toeing the line between routine military posturing and escalating provocation.

On Monday, US F-22 fighter jets intercepted two Russian bombers in international airspace 100 miles from Kodiak Island, Alaska. A US military official called the interaction "safe and professional."