The United States flew two surveillance aircraft over the Korean Peninsula on Tuesday amid growing concerns that North Korea could test a long-range missile as a “Christmas gift.”

The US Air Force RC-135W Rivet Joint was spotted above the peninsula every day since last weekend, according to aviation tracker Aircraft Spots.

The RC-135W flew at an altitude of 9.4 kilometers.

The aircraft is fitted with equipment that can detect signals before a missile is launched, and analyze its ballistic trajectory.

The US Air Force E-8C Joint Stars (JSTARS) reconnaissance plane was also seen flying at 8.8 kilometers above the peninsula on Tuesday, three days after its last flight, the military aircraft tracker said on its Twitter account without mentioning the specific time or region.

The airborne ground surveillance, battle management and command and control aircraft is known to be capable of closely monitoring North Korea’s missile launching sites, ground troops and equipment movements including artillery guns from altitudes of between nine and 12 kilometers.

With a wingspan of 44.4 meters, the E-8C measures 46.6 meters in length and 12.9 meters in height, and has a maximum cruise speed of 945 kilometers per hour. It can stay in air for nine to 11 hours, and has a flying range of 9,270 kilometers.

The US has flown reconnaissance planes every day over the Korean Peninsula since Dec. 19.

The fact that the aircraft flew with their location identification systems turned on suggests that the US intends to send warnings to North Korea.

“South Korean and US forces have intensified their surveillance of North Korea to prepare against its possible military provocations,” a South Korean military official said.

By Kim So-hyun (sophie@heraldcorp.com)