By John Irish and Matt Smith

DUBAI (Reuters) - Fears that Dubai's debt problems are not limited to troubled state conglomerate Dubai World battered investor confidence in the world's top oil-exporting region and sent shares across the Gulf tumbling on Wednesday.

Investors have been left in the dark since the Gulf business hub announced on Nov 25 that it sought to delay payment on Dubai World debt while it overhauls the sprawling state firm, which builds and operates everything from ports to luxury flats.

While Dubai's government has tried to ring-fence profitable businesses from the $26 billion restructuring at Dubai World, its problems have led to credit downgrades on all government-linked firms amid investor fears that state aid would not be forthcoming in times of trouble.

Moody's Investors Service said on Wednesday it was reviewing the ratings of issuers related to the government not just of Dubai, but of neighboring emirate Abu Dhabi and the federal government of the seven-member United Arab Emirates federation.

Dubai and Abu Dhabi are both part of the UAE, the world's third-largest oil exporter. Banks lent Dubai government-linked firms on the implicit understanding that they were backed by the federal government or Abu Dhabi, home to most of the UAE's oil.

"International banks are not going to see this as a Dubai World problem. They will see it as a state problem, a regional problem," Keith Edwards, head of asset management at Doha-based firm The First Investor, said.

Dubai World unit Nakheel, the developer whose debt is at the center of the crisis, has a $3.5 billion bond maturing Monday. The bond's trustee, Deutsche Bank, held a conference call on Wednesday to coordinate the administrative process, sources said, but it was not clear what would happen next.

"Refinancing for anything with Dubai's name on it is now going to be very difficult," said David Butter, director for Middle East and North Africa at Economist Intelligence Unit.

"The crisis has exposed the difficulty Dubai authorities will have in trying to cordon off ... real problem areas."

WHO ELSE IS IN TROUBLE?

Ratings agencies said downgrades could accelerate a payment clause for $2 billion in debts at Dubai's power and water firm. A spokesman at the utility firm, DEWA, rejected the notion.

But, underscoring potential problems for other Dubai firms, a unit of Dubai Holding, which belongs to the emirate's ruler, sold its stake in Egyptian investment bank EFG Hermes <HRHO.CA>.

Analysts said the move highlighted Dubai Holdings' vulnerable position as most at risk of defaulting on debt after Dubai World as capital markets become reluctant to lend Dubai.