United Airlines planes are seen on a platform at the Newark Liberty International Airport in New Jersey, July 8, 2015. REUTERS/Eduardo Munoz/Files

United Airlines has apologised to the owners of a dog who died in an aircraft cargo hold during a flight from Houston to San Francisco, the latest death of an animal under the airline's care.

ABC News reported that the Rasmussen family's 5-year-old King Charles Spaniel Lulu was aboard the United plane on Sunday when it was delayed on the tarmac for two hours before taking off.

The family said the dog died in the cargo hold and had been cleared to fly by a veterinarian days before the flight, ABC said.

"We are so sorry to learn of Lulu's passing and have reached out to our customer to offer our condolences and assistance," United said in a statement on Monday. "We are conducting a thorough review of this incident."

US Department of Transportation data shows that United had nine of the 26 deaths of animals reported last year during air transport, more than any other airline.

United's handling of animals drew headlines this spring when a giant rabbit named Simon died during a trip from Britain to the Iowa State Fair. Investors in the animal have sued United, claiming its workers were responsible for its death.