WASHINGTON, July 30 (Reuters) - U.S. Transportation

Secretary Pete Buttigieg on Tuesday warned the 10 largest U.S.

airlines they must comply with new passenger refund obligations

passed by Congress in May, rejecting an industry request for

more time.

Buttigieg said in letters to airline CEOs that they must

follow the new refund provisions taking a hard line with the

industry.

"The law did not provide for extensions or delays.

Accordingly, we will be taking all appropriate steps to

responsibly enforce these provisions using our investigative and

enforcement powers," Buttigieg said.

Airlines for America, an industry trade group representing

Delta Air Lines ( DAL ), United Airlines, American

Airlines ( AAL ), Southwest Airlines ( LUV ) and others, on July

3 had sought additional time on a number of provisions.

The group said "the new technology solutions necessary to

comply with these requirements will necessitate thorough

development, testing and refinement before successful

deployment."

The group did not immediately comment on Buttigieg's letter.

The DOT announcement came a day after the 5th Circuit U.S.

Court of Appeals blocked new DOT airline fee disclosure rules

from taking effect in October.

A4A warned that giving automatic refunds if consumers do not

respond to an offer of alternative transportation and the flight

departs without that passenger could "negatively impact many

consumers" and leave them without a new flight.

"Airlines must clearly and proactively inform passengers of

their right to a refund whenever their flight is canceled or

significantly changed," Buttigieg said. "Cash refunds - not

vouchers or credits - must be automatic for passengers when they

are owed."

The issue has gained new attention after a software update

by global cybersecurity firm CrowdStrike triggered system

problems for Microsoft customers including airlines on July 19.

Delta suffered widespread woes for six days, resulting in

more than 6,000 flight cancellations, impacting more than

500,000 passengers and prompting DOT to open an investigation.

Congress in May required airlines to take a number of

immediate steps including make credits good for five years.

A4A said most airlines said "they do not have the

technological means to issue and support vouchers and flight

credits with a minimum travel validity period of five years" and

many airlines "estimate that 12 months will be needed to achieve

the required solution."

Buttigieg rejected that: "Any airline that has provided

vouchers or credits with a shorter validity period must take

immediate steps to remedy this noncompliance," he wrote.