Police in the US are searching for the armed man suspected to have killed six people and seriously wounded another in three towns near the city of Philadelphia.

Pennsylvania police said they were concentrating their search for Bradley William Stone in and around Pennsburg, in Montgomery County on Monday, 80km northwest of Philadelphia.

Risa Vetri Ferman, Montgomery county's district attorney, said Stone was being sought in the shooting deaths of six people and the serious wounding of another, but authorities were unclear of his exact location.

"As I stand here right now, we do not know where he is,'' Ferman said at an evening news briefing.

The fatal shootings, including his ex-wife Nicole and her teenage niece, occurred before dawn at properties within a few kilometres of each other.

Stone is believed to have started the killing spree in Souderton, killing his ex-wife's sister, husband and 14-year-old daughter, the district attorney said.

A 17-year-old son survived but suffered a head wound and was hospitalised.

About an hour later, police found two dead women - Stone's former mother-in-law and former grandmother-in-law, in Lansdale.

Nicole Stone was later killed at her home in Harleysville. Neighbours reported hearing breaking glass and
gunshots from her apartment. They said they saw Stone fleeing with their two children and alerted authorities.

Stone left the two children with a neighbour and they were safe, Ferman said

The local school district, citing police activity in the area, had earlier ordered students and teachers to seek shelter until further notice.

Source: Agencies