A Bombardier spokesman said “a number of airlines are expressing serious interest and confidence in the aircraft’s abilities”. He added that “momentum” was increasing on sales.
Sales to Delta would be a major boost for the Canadian-owned maker’s long-delayed flagship passenger jet.
“We actually think at the right price it’s quite a competitive airplane, particularly given the engine technology,” Delta’s chief executive Richard Anderson said.
“We’re taking a very serious look at it.”
Bombardier’s production of wings for the long-overdue CSeries is Northern Ireland’s biggest-ever inward investment programme, worth £520m.
Last year the company received a $1bn (£660m) bailout from the Quebec regional government.
Bombardier currently has 243 orders for the CSeries. Its target is 300.
A spokeswoman for the firm said: “There is a high level of customer interest in the CSeries, particularly since the CS100 gained certification in December.
“Bombardier is pursuing a range of sales campaigns in the full knowledge that they will materialise over time as airlines assess their fleet requirements and make their purchase decisions. Any discussions with potential customers are confidential.”
Craig West, editor of Airliner World, said: “I think the Delta story has real substance.
“While it already operates both the Boeing 737 and the Airbus A320 families, it also has huge numbers of McDonnell Douglas MD-80s.
“They were delivered between 1988 and 1993, so they’re pretty long in the tooth.”
Bombardier’s east Belfast operation is trying to cut costs by a fifth over the next two years.
Staff recently rejected a two-year pay freeze for the 5,500 workforce and plans to extend the working week to 37 hours.
Earlier this year Bombardier said it was cutting around 60 temporary jobs here.
In May last year, it was revealed it was shedding 220 jobs, following the loss of 130 posts in February.