Police have evacuated about 80 people from their homes in a stand-off with a man reported to be in possession of “dangerous items”.
A 300m cordon has been set up in Wood End Lane, Northolt, north-west London, as armed police and negotiators speak to the man.
Officers were called to the address at about 01:00 BST on Friday over concerns the man had hazardous materials.
Emergency services have not attempted to enter the property.
A Met Police spokesperson said the force believes there is one man inside the house.
“It is not known what the man has inside the property” but “attempts have been made by him to prevent access to the address.”
“There is no suggestion that this incident is terrorist-related nor is it linked to any other ongoing investigation or operation,” the spokesperson added.
The BBC reporter Ayshea Buksh, who is at the scene, said neighbours told her the man has “aggressive, dangerous dogs” which is one of the reasons why police are being particularly cautious.
A woman claiming to be the cousin of the man involved in the stand-off has told the BBC he recently had “a breakdown”.
The man is believed to be in his 40s and according to several local residents, is regularly seen walking his four Rottweiler dogs.