Unionized workers and management at BHP Billiton Ltd.'s (BHP, BHP.AU) Spence copper mine in Chile agreed to extend the so-called "goodwill" government-assisted mediation period an additional three business days, a union leader said Wednesday.

If both sides can't hammer out a new contract by the end of the mediation period Monday, the 560 union members could go on strike early Tuesday.

"Our objective, as union leaders, is to reach a strong agreement and that's why we opted to prolong the goodwill mediation period," Francisco Aravena of the Spence union told Dow Jones Newswires.

Contracts expired Sept. 30 without both sides reaching an agreement and mine management sought the five-business-day government-assisted mediation period following local labor laws. Business days include Saturdays.

Aravena said workers will likely vote on a new offer Monday.

The union wants a 5.5% wage increase, improved benefits and bonuses totaling 15 million Chilean pesos ($27,100). Earlier this week, the mining company sweetened its offer, Aravena said, but declined to discuss the details of the new offer.

The open-pit Spence mine, which came on line a few years ago, produced about 165,000 metric tons of copper cathodes last year.

This is Spence's second collective-bargaining process as workers. The first was negotiated in 2006 before the mine went into production.

BHP also owns the Cerro Colorado mine and controls and operates the Escondida copper mine in Chile. The latter will begin wage negotiations later this year. BHP reportedly seeks to begin these talks ahead of the legally mandated schedule to avert a strike.

In 2006, when copper prices were booming, Escondida workers went on strike for nearly a month, bringing the world's largest copper mine to a standstill as they sought higher wages and production bonuses.

-By Carolina Pica, Dow Jones Newswires; 56-2-820-4244; carolina.pica@dowjones.com