There are fears the accident may have been caused by a bird strike at the village of Cley-next-to-the sea after it came down in an area popular with migrating birds.
A 400 metre cordon has been placed around the scene by emergency services amid reports the search and rescue helicopter may have been carrying limo ammunition.
The crash happened in the peaceful former port which lies not far from the RAF Lakenheath air base on the Norfolk, Suffolk border.
The predominantly flat area is widely used by the military for training sorties with one resident claiming to have seen military helicopters flying just 200ft above the ground.
Local residents said there were many fire engines at the scene and police cars were in attendance at Cley and the village of Salthouse further down the coast.
The Pave Hawk is a modified version of the famous Blackhawk helicopter and is used by the US Air Force. It is used in search and rescue operations and has a top speed of 361kph top speed.
Local resident Brian Egan, who saw the helicopters flying low shortly before the accident at 8pm said: "I saw two military helicotpers flying very low over my property.
"They were flying extremely low. We get that from time to time, but they were flying extremely low for that time of night. A couple of hours later I heard that there had been an accident."Helen Terry, 43, from Salthouse, near Cley, said: "We heard the helicopter fly over. We assumed it was just heading out to sea for training exercises. It's a daily occurrence and we're quite used to it.
"We live less than half a mile from where it's happened and we didn't hear any bang. The first we heard was when we saw emergency crews rushing to the area.
"It's something locals are used to and we've never had any safety concerns."
The 48th Air Wing of the US Air Force tweeted: "One of our HH-60G Pave Hawk helicopters was involved in an incident during a training mission."
Norfolk Police said: "Police are currently dealing with a single helicopter crash in the Cley area, on the North Norfolk coast. There are believed to be four fatalities. Officers are on the scene, with a 400 metre area cordoned off."
A spokesman for the Royal National Lifeboat Institution said: "We were asked for three lifeboats to respond to reports that an aircraft had possibly ditched in the sea.
"Lifeboats Wells, Sheringham and Cromer were launched at the request of the coastguard but were stood down when it was confirmed that the aircraft had come down over land."
RAF Lakenheath said on its website: "The primary mission of the HH-60G Pave Hawk helicopter is to conduct day or night operations into hostile environments to recover downed aircrew or other isolated personnel during war."