Cape Town - The National Union of Metalworkers of South Africa (Numsa) has rejected the latest employers' offer of a 10 percent wage increase this year and 9.5 percent in 2015 and is extending its 10-day strike, two union sources told Reuters on Friday.

Around 220,000 Numsa members have downed tools since July 1 demanding pay rises of 12-15 percent.

The strike has dealt a further blow to Africa's most advanced economy, which is still reeling from a five-month stoppage in its platinum mining sector.

The rand weakened after the news.

“We are still open for engagement with the employer and the strike is continuing,” a senior Numsa official told Reuters.

The union's secretary general and national spokesman did not respond to requests for comment.

The sector's main employers' group, the Steel and Engineering Industries Federation of South Africa, said it was yet to receive an update from NUMSA.

The walkout has hit components factories supplying US car manufacturer General Motors' South African plant, which has been forced to stop production for a week.

A four-week strike last year by more than 30,000 Numsa members at major automakers cost the industry around $2 billion. - Reuters