UPDATE: Boston bombing suspect believed surrounded: Media

WATERTOWN, Massachusetts (AFP) - Police surrounded a district where a Boston marathon suspect was believed to be hiding late Friday after a burst of automatic gunfire, media reports said.

Police squads and armoured personnel carriers raced to the district where the suspect was believed to be hiding in a boat in the back garden of a house.

Police helicopters buzzed overhead and bomb squad vans and ambulances were in position around the house at 63 Franklin Street in Watertown.

The Boston Globe quoted a source as saying police had 19-year-old Dzhokhar Tsarnaev "pinned down".

CNN said police believed they had the suspect surrounded.

Gunfire erupted in the district only minutes after police told a press conference they believed that Tsarnaev had escaped a mass dragnet thrown over Watertown.

Chechen brothers Dzhokhar and Tamerlan Tsarnaev are the main suspects in the Boston marathon bombing on Monday in which three people were killed and about 180 injured.

Tamerlan, 26, was killed in a shootout with police in the early hours of Friday after an earlier gunbattle in which a police officer was shot dead.