Multi-metal mining giant Anglo American is rumoured to be considering an offer for US coking coal miner Walter Energy.According to The Times, the FTSE 100 miner could pitch an offer price at $120 per share for Walter, a massive 60% premium to its closing share price of $74.99 on Tuesday on Wall Street. This would value the US firm at around $7.49bn.Anglo's shares were 1.97% higher at 2,430p by 11:36.Reports also suggest that sector peer BHP Billiton may also interested in a bid for Walter. BHP was trading 0.81% higher at 1,993.5p.The rumours follows heightened speculation over M&A activity in the coal sector.Anglo was also said to weighing up whether to disrupt Peabody Energy Corp's $5.1bn takeover of Australian miner Macarthur Coal with a bid of its own, says Bloomberg. However, two people close to the situation have said that the company has decided against an offer.Meanwhile, mining and commodities trading company Glencore announced on 1 September that it is offering to buy one of the largest producers of thermal coal in South Africa, Optimum Coal Holdings. In what is termed an "expression of interest" Glencore is offering to pay $1.2bn or 34 rand a share, a 35.9% premium on Optimum Coal's share price on 16 August. BC