An 18-year-old woman and 20-year-old man were killed and nine people were still hospitalised with gunfire wounds on Sunday afternoon, including a few in critical condition, police told reporters.

Among the injured were some 14 victims under the age of 18, said Richard Worley, acting commissioner of the Baltimore Police Department, noting that they were still trying to confirm victim ages.

Worley said police were still searching for multiple suspects, and he urged the community to come forward with any information or videos.

“We know for sure there are more than one. We don’t know how many (suspects),” Worley said.

The tragedy rattled the city of Baltimore, 64 km north of Washington DC, at the start of the Fourth of July holiday weekend, when Americans typically gather for parades, barbecues and fireworks.

The shooting occurred shortly after midnight at the Brooklyn Day block party in the Brooklyn Homes neighbourhood of Baltimore, which hundreds of people attended, local media reported.

A witness told TV station Fox 45 that they had heard 20 to 30 shots fired.

Television footage of the crime scene on Sunday showed police tape blocking off yards littered with cups, plates and other trash from the event.

-AAP