By Wendell Roelf CAPE TOWN (Reuters) - South Africa's NUMSA manufacturing union will continue a three-week strike after its regional branches accepted a wage offer from employers but rejected conditions relating to future pay negotiations, two union sources said on Friday. More than 200,000 National Union of Metalworkers of South Africa (NUMSA) workers downed tools on July 2 to demand higher wages, halting production at carmakers including General Motors and affecting construction at new power plants. This week, the main employer body, the Steel and Engineering Federation of South Africa (SEIFSA), "reluctantly" agreed to raise salaries by between 7 and 10 percent following a government proposal to break the impasse. However, in its offer SEIFSA insisted on a tightening of a clause known as Section 37 that is meant to prevent unions "double-dipping", or pressing for new demands at a factory or company level in spite of an existing industry-wide wage deal. "The strike will continue because we have problems with changing Section 37. They want to make the union weak and toothless," one union source told Reuters. Another source said NUMSA was open to meeting with employers over the weekend to try and broker an end to the strike, which is crimping growth in Africa's most advanced economy. NUMSA officials declined to comment, saying their official position would be articulated at a news conference on Sunday. The metals and engineering strike came soon after the end of a five-month walkout in the platinum sector, the longest and costliest strike in South Africa's history.