U.S.-based airlines and their pilots are waging a new battle against a foreign carrier they contend is taking advantage of provisions in international law to unfairly compete on American routes.

The latest lobbying fight centers on an expansion of Norwegian Air Shuttle ASA, an Oslo-based company that started out as a regional carrier but has spent much of the past decade rebuilding itself as an aggressive low-cost airline with wide reach across Europe. More recently it has begun pushing into Asia and the United States, staffing those international flights with Thai flight attendants who are paid about $500 a month, far less than the wages earned by its Norway-based attendants.

The airline, which operates under the trade name Norwegian, locked down a fleet of ultramodern Boeing 787 Dreamliners to expand its international offerings to Thailand and the United States but is registering the aircraft in Ireland, where looser labor regulations allow foreign flight attendants to work for less.

The airline’s aggressive cost-cutting has made routes between the United States and Norway’s capital, Oslo, among the most competitive international airfare offerings in North America. Through the winter, the airline is offering fares as low as $391 round-trip from New York including all taxes, undercutting United, its only U.S. competitor on the direct route, by nearly $300.

Norwegian labor unions launched a boycott campaign last fall, arguing that underpaying the foreign flight attendants threatens to undermine the workplace and wage protections that native workers enjoy. The airline responded that its Thai workers are paid salaries that are well above average for their homeland, and receive bonus payments for long shifts and layovers in expensive foreign destinations.