Along with smaller purchases in the previous trading days, Tata increased its stake by 2.92%, while Brazilian steelmaker Cia Siderurgica Nacional spent more than $76 million to lift its stake by 1.29% to 17.58% almost a month ago.

The two steelmakers collectively hold 44.72% of Riversdale, while Rio is in the running for at least a 16.99% stake based on the acceptance notifications received under its institutional acceptance facility.

Riversdale’s flagship Benga coking coal mine in Mozambique is a joint venture development, with Tata owning 35%.

At full production under stage 2 development, the Benga mine aims to export 6 million tonnes per annum of prime hard coking coal and 4Mtpa of export thermal coal by 2013.

Riversdale is also advancing a prefeasibility study for its nearby and bigger $2 billion Zambeze hard coking coal project, which holds more than 9 billion tonnes of resources.

Last year the company started investigating the long-term possibility that Zambeze could become a 90Mtpa run-of-mine operation.

Rio’s offer is expected to close at 7pm AEDT on March 14, but this deadline could be extended.

The offer is subject to a greater than 50% minimum acceptance condition, Foreign Investment Review Board approval and other conditions.

Tata is yet to make a decision on Rio’s offer, but steelmakers are increasingly concerned with securing supply of metallurgical coal as the commodity remains in strong demand.

Yesterday Riversdale shares closed down 3.4% to $14.97 while Rio shares closed down 1.2% to $84.12.