Low-cost carrier Norwegian Air has put on hold talks to buy 20 new Dreamliners from Boeing because of a delay in receiving U.S. Department of Transportation approval for its long haul plans, Reuters reported Monday.

The airline has launched flights from Europe to Asia and the U.S. based on a license issued by non-European Union member Norway, but has applied to fly to the U.S. on an Irish license, wanting to take advantage of wider traffic rights offered by the so-called open skies agreement between the EU and the U.S.

American labor unions and competing airlines have voiced strong opposition to the plan, which in part cuts costs by using Asian crews, and the U.S. Department of Transportation has yet to decide on Norwegian's application.

In an April 23 letter to Irish transport minister Leo Varadkar released Monday, Norwegian Air Chief Executive Bjoern Kjos asked for "assistance and support in order to find a solution" with the U.S. Department of Transportation.

"Unfortunately, the delay in the DOT process has given us no other choice than to put our ongoing negotiations with Boeing to purchase 20 new 787-9 Dreamliner aircraft - due to be registered in Ireland - on hold until Norwegian Air International's future in the U.S. has been decided," Kjos wrote.