Assistant Chief Constable Terry Sweeney of the North-West Counter Terrorism Unit said the force's "main priority is to safely recover" the munitions that went missing from a freight train between Cumbria and Oxfordshire.
Asst Chief Con Sweeney said:
At this stage there is nothing to suggest the theft is terrorist-related, but due to the potential complexity of the investigation, our enquiries are being led by counter terrorism officers.
We would like to appeal to any members of the public who may know the whereabouts of these items, or any persons involved in the theft of these items to contact the police immediately.
I want to make it clear that, in their normal condition, these munitions are stable and do not pose a risk to the public.
However, the materials could be hazardous if they were to be tampered with and we therefore ask that anyone who sees them or knows where they are to call the police as soon as possible.
He urged anyone with any information to contact the police or Crimestoppers on 0800 555 111.
More top news
The Novichok incident in Salisbury has turned into an international diplomatic crisis. But how bad could things get? Adam Shaw reports.
Among those murdered was Keith Palmer, an unarmed constable who "did his job and never wanted any fuss."
Top temperature 14 Celsius (57 F).