UPDATE 2-Two missing in blast at US ConAgra food plant

* Explosion causes roof to collapse, blows out wall

* 20 hospitalized, 2 missing at North Carolina food plant (Recasts, adds details)

WILMINGTON, N.C., June 9 (Reuters) - An explosion caused the collapse on Tuesday of parts of the roof at a ConAgra Foods Inc (CAG.N: Quote, Profile, Research) processing plant in Garner, North Carolina, injuring at least 20 people and leaving two others missing, police said.

About a dozen people were exposed to toxic fumes and others suffered burns, local news media reported. The plant employed 900 people in three shifts and made Slim Jim meat sticks and potato chips, the Raleigh News and Observer said on its website.

A Garner police officer, Sergeant Joe Binns, said 20 people were taken to local hospitals but there were no reports of any deaths so far. Two workers remained unaccounted for, he added.

"It is an active rescue scene," said Captain Bud Davenport of the fire department in Garner, a town located just south of the state capital Raleigh.

The explosion started a fire and blew out a wall that fell onto a row of parked cars, crushing them. People at the scene reported a strong smell of ammonia fumes.

Television footage showed rescue workers in hazardous-materials suits at the plant, where the roof collapsed in several places. (Reporting by Bob Burgdorfer in Chicago, Pascal Fletcher and Jane Sutton in Miami; Editing by Pascal Fletcher and Will Dunham)