The three-month extension for Sihag, who is holding additional charge of CMD, is effective from April 1.
He also holds the post of Additional Secretary and Financial Adviser in Steel Ministry.
"In terms of order dated April 4, 2016 of Steel Ministry, the additional charge of the post of CMD NMDC is assigned to Bharathi S Sihag for a further period of three months with effect from April 1, 2016," NMDC said in a regulatory filing.
The previous CMD of the country's largest iron ore miner Narendra Kothari retired on December 31.
Government's head hunter Public Enterprises Selection Board (PSEB) recommended Gopal Singh for the top post, but sources said that Appointments Committee of the Cabinet has still not given its nod on the same.
Gopal Singh has been heading Central Coalfields Ltd (CCL) since March 2012.