The mining heavyweight announced that its interest in the South African coal producer had reached 49.49%.

The 50c boost to the offer applied if Rio secured more than 47% of acceptance notifications under its institutional acceptance facility by 7pm on April 6.

The offer period has been extended for two more weeks as a result of the offer price increase.

The offer will now end at 7pm AEST on April 20.

Rio urged Riversdale shareholders who were yet to accept the offer to do so as soon as possible.

Tata Steel and Brazilian steelmaker Cia Siderurgica Nacional both recently increased their stakes in Riversdale and collectively hold 44.72% of the company.

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

At full production under stage-two development, the Benga mine is aimed 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.

Riversdale shares were unchanged at $16.42 in morning trade while Rio was unchanged at $86.10.