LOS ANGELES (AFP) - United States (US) aviation authorities have fined South Korea's Asiana Airlines US$500,000 (S$633,000) over last year's deadly San Francisco air crash for failing to properly help families of victims, officials said on Tuesday.

Three passengers died when Asiana Airlines Flight 214 clipped a seawall with its landing gear, skidded off the runway and burst into flames at the end of an otherwise routine flight from Seoul to San Francisco on July 6.

Asiana "violated federal law last July by failing to adhere to the assurances in its family-assistance plan following the crash", the US Department of Transportation said.

The fine is the first imposed by the DoT under a 1997 law which requires foreign airlines to keep to a "family-assistance plan" in the event of aircraft accidents resulting in a major loss of life, it said.

The DoT said Asiana took two days to send enough trained staff and failed to widely publicise a helpline telephone number for families, some of whom were not contacted for up to five days after the crash.

"In the very rare event of a crash, airlines have a responsibility to provide their full support to help passengers and their families by following all the elements of their family assistance plans," said US Transportation Secretary Anthony Foxx.

"The last thing families and passengers should have to worry about at such a stressful time is how to get information from their carrier," he added.

One of the three victims had been pulled alive from the plane and placed near one wing. But the Chinese teenager was later run over and killed by a fire truck, which did not spot her lying under a layer of fire retardant foam.

US prosecutors decided in October not to bring charges against the firefighter driving the truck.

Another 182 passengers and crew aboard the Boeing 777 were injured, in the first fatal commercial airline crash in the US since 2009.

The DoT said that Asiana initially lacked adequate staff to communicate in the languages spoken by the flight's passengers.

On the telephone number response, it said.

"The only number generally available to the public that family members could call was Asiana's toll-free reservations line. Locating this phone number on Asiana's website required significant effort," it said.

The reservations line did not include a separate menu option for calls related to the crash and callers were required to navigate through cumbersome automated menus before being connected to an Asiana employee.

"In addition, Asiana took two full days to successfully contact the families of just three-quarters of the passengers. The families of several passengers were not contacted until five days following the crash.

"Asiana's response to the crash of flight 214 indicates that the carrier failed to commit sufficient resources to carry out its family-assistance plan," it said.

A final US report on the reasons for the crash is expected to be ready by the first anniversary of the disaster.