Later, a spokeswoman for Great Yarmouth coastguards said five people had been recovered.
The helicopter was bringing workers from a North Sea gas field when it ditched about 30 miles off Cromer, she said.
The helicopter is believed to be owned by the firm Bristow.
The spokeswoman said a rescue helicopter from a military base in Wattisham, Suffolk was in the area along with the Cromer Lifeboat and other vessels.
Rescued casualties had been taken to The Santa Fe Monarch oil rig, the MoD said.
mfl