Heavy firing was reported around the U.S. Embassy in this central African country yesterday as warring political groups broke a cease-fire just hours after it was signed.

The renewed fighting between the private armies of President Goukouni Queddei and Defense Minister Hissein Habre delayed the scheduled evacuation of American diplomatic personnel, including Ambassador Donald R. Norland.

The fighting began early Saturday with intense artillery duels. It constituted the most serious breach yet of a peace agreement signed last August among Chad's 11 main armed political factions. The latest clashes were believed to have caused many civilian African casualties and heavy material damage to the capital as the two sides turned the city into a battle zone.

The U.S. government ordered 20 of its diplomats to leave Chad, and the American ambassador ordered all 12 private Americans living in this capital of 400,000 population to leave as well.

In Paris, a presidential spokesman said France's approximately 1,100 troops in the former French colony were under strict orders not to get involved in the fighting. Instead, they were protecting the capital's 700 European residents, many of whom took shelter in the French Embassy.

The peace pace in August ended 16 years of civil war after a final bloody month in which 5,000 people were reported to have been massacred.