United Airlines (UA, Chicago O'Hare) has announced that it will close three of its four foreign crew bases as of October 1, leaving London Heathrow as its sole non-US station going forward.

The airline said in an internal memo seen by Reuters it could not to sustain bases at Frankfurt Int'l, Tokyo Narita, and Hong Kong Int'l in the current market environment. All 840 affected employees will be offered a transfer to one of the carrier's US bases before October, subject to their eligibility to work in the United States.

"While no other base changes are anticipated [at] this time, we'll continue to evaluate the viability of all locations based on the network schedule, government travel restrictions and restoration of demand," head of inflight services John Slater said in the memo.

The terms of US federal payroll support package under the CARES Act prohibit carriers that benefit from the assistance from any forced lay-offs before October. The airlines may, and have, engaged in voluntary redundancies.

United's only base outside of the US mainland is at Guam Int'l, a US island territory in Micronesia.