Citing an unnamed senior administration official, US media reports have said that Obama was set to announce Hagel's nomination on Monday.
Hagel has been criticized for his past comments on Israel and the sanctions on Iran aimed at dissuading Tehran from its nuclear program.
Obama was also expected to announce his pick for a new director for the Central Intelligence Agency.
For the CIA helm, Obama reportedly was considering Michael Morell, who currently heads the agency temporarily since General David Petraeus resigned last November citing an extramarital affair, and John Brennan, Obama's current terrorism and intelligence advisor.
