The US has opened an investigation into Delta Airlines as it struggles to recover from last week's global IT outage.

Transportation Secretary Pete Buttigieg said Delta must provide passengers with refunds and other compensation for disrupted travel as required by law.

A day earlier, Delta, which has cancelled more than 5,000 flights since Friday, signalled it could take several more days for operations to return to normal.

Meanwhile, CrowdStrike's boss George Kurtz, whose cybersecurity firm sparked the chaos with a faulty overnight update, has also been called to testify in Congress.

The House Committee for Homeland Security said in a letter: "While we appreciate CrowdStrike’s response and coordination with stakeholders, we cannot ignore the magnitude of this incident.

It added: "Recognising that Americans will undoubtedly feel the lasting, real-world consequences of this incident, they deserve to know in detail how this incident happened and the mitigation steps CrowdStrike is taking.”

CrowdStrike said it was working on ways to make it easier and faster to fix the issue, which downed an estimated 8.5 million Microsoft Windows devices globally, hitting airlines, health systems and other services.

In many cases, IT teams had to reboot computers manually.

Delta, which appears to have had more difficulty recovering than other airlines, said that its crew-staffing system had been affected by the incident.

As of Tuesday morning, it had already cancelled more than 400 flights and delayed hundreds of others, following more than 1,150 cancellations a day earlier, according to tracking firm Flight Aware.

Delta said it was focused on restoring operating and "fully cooperating" with the Department of Transportation's investigation.

"Across our operation, Delta teams are working tirelessly to care for and make it right for customers impacted by delays and cancellations as we work to restore the reliable, on-time service they have come to expect from Delta,” the company said.

But the incident has raised pressure on the company.

In announcing the investigation, Mr Buttigieg said: "All airline passengers have the right to be treated fairly, and I will make sure that right is upheld."

"While you should first try to resolve issues directly with the airline, we want to hear from passengers who believe that Delta has not complied with [the transport department's] enforced passenger protection requirements during the recent travel disruptions. We will follow up," he wrote on social media.

Senator Maria Cantwell, who leads the Senate commerce committee, also sent a letter to the company, noting that it had been "far slower" than other airlines to recover.

She faulted the firm for inaccurately describing passenger rights to refunds on its website and noted its responsibility to ensure live customer service agents are available to assist customers.

"While the technology outage was clearly not caused by Delta or any airline, I am nevertheless concerned that Delta is failing to meet the moment and adequately protect the needs of passengers," she wrote.

The Biden administration in April finalised a rule requiring that airlines promptly and automatically refund passengers for significant changes to their travel, and other issues.

It was part of a broader effort to step up policing of airlines after a series of major travel meltdowns.

Last year, Southwest Airlines agreed to pay $140m (£108m) resolve a Department of Transportation investigation launched after a storm disrupted service and set off a cascade of cancellations during the busy 2022 holiday travel period.