US aviation regulators said Monday that thousands of Boeing 737 airplanes would need to be inspected, amid concerns that passenger oxygen masks could fail in emergencies.

The airworthiness directive by the Federal Aviation Administration (FAA) comes into effect immediately and affects more than 2,600 US-registered airplanes.

The aim is to ensure that passenger service unit oxygen generators are in the right position on certain Boeing airplanes.

Operators are to check the oxygen generators and "perform corrective actions, if necessary, within 120 to 150 days," the FAA said in a statement.

It added that the directive was prompted by multiple reports of these generators shifting out of position, and called for a general visual inspection to be conducted.

In response to queries, Boeing said that a new adhesive introduced in August 2019 was found to sometimes allow oxygen generator units to shift from their positions.

In June, Boeing gave operators instructions to update a portion of the restraining straps on 737 oxygen generators.

The company added that it has since gone back to using its original adhesive to ensure that the generators remain in place.

The announcement adds to a series of concerns facing Boeing.

The manufacturer has been grappling with intense scrutiny since a near-catastrophic event in January when a fuselage panel blew out of an Alaska Airlines-operated 737 MAX.

Earlier on Monday, Boeing said it had reached a deal with the US Department of Justice over two fatal 737 MAX crashes -- with court documents indicating that the aviation giant would plead guilty to fraud.

Prosecutors had concluded that Boeing flouted an earlier settlement addressing the disasters, in which 346 people were killed in Ethiopia and Indonesia more than five years ago.