South African Airways (SAA) signed a deal on Friday with a union not involved in a strike that has crippled the heavily indebted carrier, giving its members a 5.9% pay rise backdated to April.

The National Transport Movement (NTM) is one of the largest unions at SAA but its members are not participating in a strike, now in its eighth day, that has left the airline with almost no cash and at risk of missing salary payments this month.

The pay increase granted to NTM will be effective from April 2019 but NTM members would only start receiving higher salaries from February 2020. Back payments due from April 2019 would be paid in lump sums in March and April 2020.

NTM President Mashudu Raphetha told Reuters the wage deal was subject to SAA securing additional funds.

The state-owned carrier's swift decline has turned it into a black hole for government bailout money. The government has spent about 20 billion rand ($1.4 billion) on SAA in the past three years.

Union leaders at the National Union of Metalworkers of South Africa and the South African Cabin Crew Association, which are leading the strike, did not respond to requests for comment.