A fire broke out yesterday in the back yard of the South Korean ambassador's home in Northwest Washington when a World War I-era chemical agent buried there reacted with air. About 10 workers at American University and some nearby homes were evacuated.

No injuries were reported.

The fire occurred in a 20-by-30-foot steel vapor-containment structure, where the U.S. Army Corps of Engineers is conducting part of a $4 million operation to excavate chemical agents long buried in the Glenbrook Road yard.

Three U.S. Army ordnance specialists, wearing protective gear, were in the structure when the fire began at 12:38 p.m., said Maj. Brian Plaisted, site manager for the Army Corps of Engineers. Two were digging in a 10-foot-deep pit when "they moved some soil and saw the smoke coming out of the ground," Plaisted said.

Engineers aren't sure whether the digging broke the chemical agent's container or it had rusted, mingling the agent with air when the diggers turned over the soil, Plaisted said.

Either way, Plaisted said, crystals of the agent, a chemical compound called white phosphorus, made contact with the air. "It started smoking initially, and then after a couple of minutes it started flaming," he said.

D.C. fire and police units decided to let the fire burn out because it was in the vapor-containment structure, Plaisted said. The fire subsided in about two hours.

"It's not serious at all; there's a long distance between the house and the site," said the wife of Ambassador Hong-koo Lee. "The family's fine." Four other homes on Glenbrook Road were evacuated, said D.C. police Capt. Anthony Poteat.

Karen Egbert, who lives near the embassy residence, was home with her two daughters and a friend of theirs when police asked them to evacuate.

"We all went to Starbucks and we all feasted on Frappuccino," Egbert said. "How's that for a real middle-class reaction to a chemical alert?"

Soldiers have used white phosphorus to create a wall of flame and smoke to provide cover, Plaisted said. The chemical agent is still used by armies.

The Army excavation, called Operation Safe Removal, began after an investigation concluded last year that the embassy residence's back yard contains two pits--including the one where the fire broke out yesterday--where soldiers at what was then the American University Experiment Station buried canisters of chemical agents.

Staff writer Peter Pae contributed to this report.