By Tom Sims

FRANKFURT: While Ukraine's airspace has been largely empty amid the crisis with Russia, a remotely piloted U.S. military vehicle called the RQ-4 Global Hawk has flown over the country in circles for hours at a time.

Over the past month, two of the spy planes have travelled on regular missions from the Mediterranean Sea to Ukraine, where they have navigated in repeated loops in the north and the east, according to Flightradar24.

The drones' high-altitude, long-distance flights have coincided with a military build-up by Russia along the Ukrainian border and a flurry of diplomacy among leaders of the United States, Europe and Russia to avert war.

At times, the two planes - under the call signs Forte10, Forte11 and Forte12 - have been the only active aircraft publicly visible over eastern Ukraine. Aviation watchers have taken note, speculating that the United States is making its presence known in a show of force.

"With these types of flights, leaving the transponder on is a conscious decision," said Ian Petchenik, communications director at Flightradar24.

The U.S. Air Force declined to comment on the details of the flights but said the United States routinely operates aircraft in support of intelligence objectives.

"These missions demonstrate our continued commitment to safety and security in the region," a spokesperson said.

On Monday evening, Forte11 returned to the Mediterranean after a nearly 24-hour trip over Ukraine. Its transponder went off at a low altitude near the Sigonella Naval Air Station in Sicily. It was the 13th similar mission.

According to the Air Force's website, the mission of such aircraft is to collect intelligence, surveillance and reconnaissance "to support joint combatant forces in worldwide peacetime, contingency and wartime operations."

Their manufacturer, Northrop Grumman, says the planes see "potential threats" and "gather near-real-time, high-resolution imagery of large areas of land in all types of weather – day or night."

The flight path has stood out because international commercial aviation, out of an abundance of caution, has largely avoided Ukraine airspace, especially in the east along the Russian border.

On Feb. 15, Flightradar24 saidForte11 was its most tracked flight, and then later reported that it had been airborne for more than 21 hours.

The aircraft, with a wingspan of nearly 40 meters and length of 15 meters, can typically fly more than 30 hours.

In 2014, the craft travelled 34.3 hours without refueling to set a record for the U.S. Air Force, according to its website.

Northrop Grumman has been providing the aircraft to the USAF for 20 years. In August, it won a five-year contract to maintain the craft for NATO.