MONTREAL -- Bombardier crossed the landmark order of 500 planes for both its Q400 turboprop and C series aircraft on Wednesday as it revealed three new agreements with international carriers.

At the Farnborough Airshow in Britain, the Canadian manufacturer said it has secured tentative interest from two unnamed buyers for up to 18 CSeries aircraft, but continues to await confirmation of new firm commitments for its new commercial airliner.

One existing CSeries customer placed a conditional order for seven CS300 aircraft valued at US$553 million, and purchase rights for six more planes.

The seven aircraft have production slots and the order is expected to be firmed up after the buyer's board of directors approves the purchase, said spokeswoman Marianella de la Barrera.

Another Bombardier customer hopes to become the first African CSeries operator after signing a letter of intent for five of the aircraft under development, valued at US$365 million at list prices.

To date, Bombardier (TSX:BBD.B) has received commitments for 513 CSeries aircraft, including firm orders for 203 of the 110- to 160-seat aircraft.

De la Barrera said the CSeries is gaining momentum and the manufacturer is not disappointed to have been unable to announce a new firm order at the Farnborough Airshow.

"For the CSeries aircraft right now we seem to be moving at a solid pace and Farnborough has been a great show for us," she said in an interview.

Two of the customers are seeking to add more planes and a third confirmed the exercise of options.

Sales and marketing vice-president Ray Jones said the company is very optimistic about the CSeries orders and is "well on our way" to reaching its target of 300 firm orders by the time the plane enters into service.

Test flights of the CSeries since have been grounded since the end of May when "an engine-related incident" damaged a test aircraft during maintenance testing on the ground.

Engine manufacturer Pratt & Whitney has said the problem was a seal issue within the oil system and Bombardier expects test flights to resume within weeks after a fix is completed.

The Montreal-based company also announced Wednesday that Thailand's low-cost airline Nok Air converted two Q400 NextGen purchase rights to firm orders, worth US$65 million.

Nok Air is the launch customer for the new 86-seat version of the turboprop. It has converted half of its four purchase rights from November 2013. It also has four firm orders.

Airline CEO Patee Sarasin said the Q400 is central to its growth strategy.

"The Q400 NextGen airliner will play a major role in our domestic development, but also in our international expansion as we look at new destinations such as Myanmar, Laos and Cambodia -- areas with airports not serviceable by jets but easily accessible with the Q400 turboprop."

In the past week, Bombardier has announced 68 commitments for the CSeries, including two conversions to firm orders. It also secured eight expressions of interest in the Q400, including two firm order conversions and one new firm order.