The public have been advised to avoid the area around Sterncourt Road as emergency services continue their response.
The force said: “Given the recent change to the UK threat level, we would like to reassure the public from the outset we are not treating this as a suspected terrorist incident.
“We ask people not to speculate around the circumstances and continue to encourage the public to avoid the area while emergency services respond to this incident.”
The College of Policing defines a major incident as “an event or situation with a range of serious consequences which requires special arrangements to be implemented by one or more emergency responder agencies”.
Superintendent Ebbs added: “We know this incident will be of great concern to those living in the affected communities, and our neighbourhood policing teams will be carrying out patrols to provide reassurance and advice.
“And if anyone is worried or concerned, please speak to an officer. This does remain a complex and sensitive investigation and we’ll continue to provide updates as and when they’re available.”