The US Department of Transportation has ordered six airlines to pay more than $600 million in passenger refunds over cancelled or changed flights, after travellers sought redress for failing to get their money back.

Authorities have received a "flood of complaints from air travellers" since the start of the pandemic, over a lack of timely refunds from companies, the department said in a statement released on Monday.

The airlines hit were American low-cost carrier Frontier, as well as foreign carriers Air India, TAP Portugal, Aeromexico, El Al and Avianca.

"A flight cancellation is frustrating enough, and you shouldn't also have to haggle or wait months to get your refund," said Transportation Secretary Pete Buttigieg.

In addition to the refunds, the Transportation Department also said it was assessing more than $7.25 million in civil penalties against the airlines, over "extreme delays in providing refunds."

The department's actions have led to "thousands of passengers" being provided with more than half a billion dollars in required refunds, the department added.

Among the airlines, Frontier faced the highest penalty at $2.2 million, while Air India was hit with a $1.4 million fine.

Under US law, airlines and ticket agents are legally obliged to refund customers if the airline cancels or "significantly changes" a flight to, from and within the United States and if the consumer does not want to accept the offered alternative.

It is unlawful for airlines to refuse refunds and provide vouchers instead to consumers, authorities said.