WASHINGTON (Reuters) - The U.S. Army on Tuesday urged Delaware residents to be cautious after three Air Force bomb disposal experts were injured by mustard gas from a World War I weapon dredged off America's coast.

The three from Dover Air Force base were hurt last month after they examined a rusty weapons shell dumped on a driveway in Bridgeville, Delaware, along with crushed sea shells used as surface cover on the drive, said a spokeswoman from the base.

The weapon is believed to be a surplus munition from World War I dumped at sea and was later identified as containing the dangerous blister agent, mustard gas.

An Army spokeswoman in Washington said the military was trying to find the exact source of the weapon and the Army Corps of Engineers had been sent to the area to investigate.

"We do realize that this is a public safety concern and so if anyone comes into contact with something like this they need to call 911. Don't touch anything," said the spokeswoman.

Cpl. Helen Zane of the Delaware State Police said rusted surplus munitions dumped at sea often turned up among sea shells used to cover driveways.

"It's really not uncommon in downstate Delaware to find mortars and other things on driveways," said Zane. "But this is the first time we have ever heard of any mustard gas."

It was standard military procedure after World War I to bury surplus munitions at sea and the U.S. military is trying to clean up those sites.

Major Cheryl Law, a spokeswoman for Dover Air Force base, said the bomb disposal team who looked at the weapon had broken its casing with an explosive charge and then noticed a dark tar-like substance coming from it.

The weapon was then put in a sealed container and sent to Aberdeen Proving Ground in Maryland where it tested positive for mustard gas.

Two days later, three members of the Air Force team began feeling ill and were taken to the hospital. They have all since been discharged.