JOHANNESBURG, July 14 (Reuters) - Workers in South Africa's gold sector rejected the latest offer for a pay raise of between 8 percent and 10 percent and vowed to soon escalate the dispute, the National Union of Mineworkers (NUM) said on Tuesday 14 July.

Three South African unions are holding talks with gold producers under the guidance of a mediator after failing to agree on wage increases. The unions have warned they could strike if negotiations fail.

"NUM has this morning rejected the Chamber of Mines' offer of between 8 and 10 percent from gold employers and will escalate the dispute on July 21," NUM spokesman Lesiba Seshoka said.

The union -- South Africa's biggest mineworkers body -- received the improved offer on Monday 13 July but said the increase might not be enough to avert a strike.

The wage talks are being watched closely by markets because a strike could affect production, mining shares or metal prices.

The Solidarity labour union said separately on Tuesday 14 July it had also rejected a fresh offer for workers it represents.

It said AngloGold Ashanti (ANGJ.J), Gold Fields (GFIJ.J) and Rand Uranium offered a wage increase of 8.5 percent for some workers, while Harmony offered a wage increase of 6.5 percent and an additional boost linked to the price of gold.

The unions have now warned that the next round of negotiations on July 21 may be the last chance for the gold firms to stop a strike.

"We've said to them we see this now as the last opportunity. If there's no improved offer by next Tuesday (21 July), we'll ask for a strike certificate," Solidarity spokesman Jaco Kleynhans told Reuters.

Mine workers, whose lowest-earning members earn 3,300 rand ($403.6) a month, have demanded a 15 percent pay rise, arguing the price of gold is up despite the global economic downturn.

But gold firms say they can't afford the increase, which is nearly twice South Africa's consumer inflation of 8 percent.

South Africa is the world's No. 3 gold producer, and the mining sector is subject to intense scrutiny by big foreign groups such as Anglo American Plc (AAL.L), South Africa's biggest mining player.

(Reporting by Agnieszka Flak and Muchena Zigomo, Editing by Michael Kahn)