The balloons took off from a launch site north of Bristol on Saturday One of the balloons taking part in the Gordon Bennett Cup is missing in thunderstorms over the Adriatic.
The missing hydrogen balloon, piloted by Richard Abruzzo and Carol Rymer Davis, is one of three US entries.
A spokeswoman at race control in Bristol said contact had been lost with the balloon, and there was concern that it might have had to ditch.
Organisers were in touch with the balloon’s ground team and Italian air traffic control, she said.
Don Cameron from race control said the last satellite tracker report from USA2 was at 0658 BST.
“The Italian Maritime Rescue Co-ordination Centre have initiated an operation with a helicopter and fast boat although no ELT [distress beacon] activation has been detected so far,” he said.
“The Croatian authorities and all shipping have been informed.
“Thunderstorm activity has been reported in the area. We are very concerned, but can only wait for news now.”
The balloons took off from a launch site to the north of Bristol on Saturday night.
The winning team is the one to travel the furthest and crew control the balloon by letting gas out to descend or throwing sandbags out to ascend.
At first light on Wednesday just three teams of the 20 that started, including the Americans, were still flying in the Gordon Bennett 2010 international gas balloon race.
A German balloon piloted by Wilhelm Eimers and Ullrich Seel is expected to land shortly in Moldova.
A British balloon piloted by David Hempleman-Adams and Steve Carey is currently looking for a landing site in mountainous terrain in eastern Serbia.
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