WASHINGTON (Reuters) - The Obama administration will seek maximum fines against airlines that violate a new rule aimed at curbing lengthy ground delays, Transportation Secretary Ray LaHood said on Tuesday.

LaHood told reporters that his agency had rejected airline requests for exemptions to the regulation spurred by outraged passengers, some of whom were stuck on planes for several hours without adequate food or restroom facilities.

"There will be strong enforcement," LaHood said. "That has to be part of the plan in order to make sure that passengers understand and airlines understand that we're serious about this."

The regulation, which takes effect on Thursday, would fine airlines up to $27,000 per passenger for tarmac delays that exceed three hours. Passengers could be let off a plane, if they request.

Airlines say the rule could lead to more flight cancellations as they seek to avoid penalties, especially when storms threaten to divert or ground flights.

There were nearly 900 ground delays of at least three hours between February 2009 and February 2010, according to the latest government figures.

The monthly total peaked last June at 278 and fell sharply through the summer and into the later part of the year. Most long ground delays occur while planes wait to take off, and most involve domestic airlines that fly hundreds of thousands of flights per month combined.

Delta Air Lines, Southwest Airlines and United Airlines, a unit of UAL Corp, all had at least two ground delays of three hours or longer in February, figures show.

American Airlines, a unit of AMR Corp, had 16 while US Airways led all carriers with 25.

There are exemptions to the rule if pilots or air traffic controllers determine that returning an aircraft to the airport gate after three hours to let passengers off would disrupt airport operations.

Oversight will be voluntary, but LaHood said his agency would be watching closely.

"I have no doubt we'll be hearing from passengers," LaHood said.

Legislation awaiting final action in Congress would make the new regulation law.

The tarmac regulation does not apply to international airlines.

(Reporting by John Crawley; Editing by Richard Chang)