The US military conducted a drill scoping out possible escape routes in NYC for Republican President-elect Donald Trump. Reuters/Lucas Jackson

Three military aircraft circling at low levels over midtown Manhattan last week were reviewing possible escape routes for President-elect Donald Trump.

The aircraft were conducting surveillance and mapping escape routes as part of a security plan, two law enforcement officials told CNN. The law enforcement sources said that an HC-130 search and rescue plane and two HH-60 Pave Hawk helicopters were tasked with taking photos and surveying rooftops and street layouts for the Secret Service.

According to New York State Division of Military and Naval Affairs spokesman Eric Durr, the aircraft flew out of the Francis S Gabreski Air National Guard Base in Westhampton Beach, New York, and were piloted by air national guardsmen from the 106th Rescue Wing.

Further details about the nature or duration of the flights were not given. The New York Post reported the drill lasted about 40 minutes.

New Yorkers took notice of the large plane and helicopters, CNN noted. "Why is this plane circulating so low in Manhattan?! It brings back bad memories," Erdal Kuyumcu wrote on Twitter.

A spokesman for New York City Mayor Bill de Blasio and the city's Office of Emergency Management (OEM) said residents did not receive notification about the security exercise by the city because key city officials were not informed. The city regularly sends residents alerts of similar flyovers, CNN reported.

OEM spokeswoman Nancy Silvestri said that it was "standing protocol" for the Federal Aviation Administration (FAA) to alert the department about low flying planes and exercises "so that we can get that information out to the public to avoid panic." Silvestri said that her department had spoken to the FAA and Secret Service so that the protocol was followed in the future.

The FAA said the Secret Service has authorised a temporary flight restriction and authorised military flights over the city on 13 December. The agency added it was the responsibility of the Secret Service, as the approval authority, "to conduct other interagency and public notifications."

Earlier in December, De Blasio estimated the cost of protecting Trump between 9 November and 19 December at roughly half a million dollars a day.