US security agents searching a checked suitcase after an alarm went off at New York's LaGuardia Airport expected the usual contraband - guns, knives or a hand grenade - but instead found a stowaway Chihuahua, authorities say.

Staring back at the agents was a 7-year-old beige and brown dog who had sneaked into the luggage, which its owner had checked onto her American Airlines flight to Los Angeles, said Lisa Farbstein, spokeswoman for the Transportation Security Administration (TSA).

"She was just as surprised as we were," Ms Farbstein said.

"The woman said that the dog must have climbed into the suitcase and curled up in the clothing while she was packing for her trip."

The discovery was made on March 3 as the luggage was headed for the plane's cargo hold, through the usual screening equipment designed to detect anything out of the ordinary.

When an alarm sounded, agents popped open the suitcase to inspect its contents and found the tea cup-sized rover.

"The officer was very surprised to see the dog," Ms Farbstein said.

TSA agents, whose past checked luggage searches have turned up such dangerous items as a gas-filled chainsaw, reunited the tiny dog with its owner, who was not identified.

The woman contacted her husband, who retrieved the pet and brought it home.

American Airlines said it did not have any information about the incident, including how cold or hot a plane's cargo hold could get.

Reuters