SANTIAGO, Jan 2 - Chile's state copper giant Codelco said on Monday it exercised a disputed option to buy a stake in Anglo American's south Chile mining assets, but is willing to consider taking a smaller stake if Anglo proposes a formula that safeguards its full value.

Anglo last year announced it had sold a 24.5 percent stake in the assets to Japan's Mitsubishi <8058.T>, which Anglo said halved Codelco's potential 49 percent stake. The move surprised world No. 1 copper producer Codelco, along with investors.

Codelco, which viewed Anglo's preemptive move as a snub, said on Monday it exercised its option to buy a 49 percent stake in the south Chilean properties of Anglo American for an estimated price of around $6 billion. At current copper prices, experts say the stake is worth far more.

Anglo responded to Codelco's move by saying it was not obliged to sell any shares in the south Chile properties to Codelco. This sets the stage for a potentially lengthy legal battle.

Anglo's properties in southern Chile include the flagship expansion project Los Bronces, where Anglo has invested around $2.8 billion; the El Soldado mine; the Chagres smelter; and the Los Sulfatos and San Enrique Monolito exploration projects.

Codelco's Chairman Gerardo Jofre said he wanted Anglo to sell the state miner the 24.5 percent stake that is not in contention now, and said it would fight for the balance.

We are going after the 49 percent, or its equivalent value in the worst case, Jofre told a news conference. The economic value can be captured in different ways.

No-one should misunderstand the fact we are willing to receive a 24.5 percent (stake), or however much is not in dispute. We are going to fight for the other part by all legitimate means, he added.

In November, Anglo American said it sold a stake of the assets in question to Japan's Mitsubishi <8058.T>. The company said this meant Codelco now had a right to buy only a 24.5 percent stake.

In December, Anglo filed a writ against Codelco accusing it of breach of contract and seeking to avoid the option altogether.

The legal status of the disputed assets was not immediately clear following the latest salvos, but a protracted legal battle looks increasingly likely.

In November, Codelco Chief Executive Diego Hernandez estimated the legal battle with Anglo could take three to four years to resolve.

Codelco says it made public its intention to exercise the option months ago. Anglo American has said it was within its right to sell a stake to Mitsubishi because Codelco's window to exercise its option only opened this month.

Codelco says Anglo American has reneged on its option contract and has acted in bad faith.

A Chilean court in December maintained a sales freeze on Anglo American's southern Chilean properties, preventing the London-listed miner from selling another part of its coveted assets after Codelco filed an appeal to safeguard them.

Anglo American Sur accounted for 41 percent of Anglo's total copper production in 2010. Analysts have estimated the south Chilean assets make up 17 percent of Anglo American's net asset value -- roughly equivalent to its platinum operations.

(With reporting by Simon Gardner and Antonio de la Jara; Editing by David Gregorio)