Kim Jong Il's son and successor was given the title at a meeting Friday of the Political Bureau of the Central Committee of the Workers' Party of Korea, the North's official Korean Central News Agency said in a statement.
Kim Jong Un "assumed supreme commandership of the Korean People's Army" according to a will made by Kim Jong Il on Oct. 8, the statement said.
Kim has received a string of titles from the government and state media in the wake of his father's death on Dec. 17. But the title Supreme Commander is a clear signal that Kim Jong Un is fast consolidating power over North Korea.
The North also warned Friday that there would be no softening of its position toward South Korea's government after Kim Jong Il's death.
North Korea's powerful National Defense Commission said the country would never deal with South Korean President Lee Myung-bak, a conservative who stopped a no-strings-attached aid policy toward the North in 2008.
The stern message also said North Korea was uniting around Kim Jong Un, referring to him for the first time with the title Great Leader -- previously used for his father -- in a clear message of continuity. It was the latest incremental step in a burgeoning personality cult around the son following the Dec. 17 death of Kim Jong Il.
The top levels of government appear to have rallied around Kim Jong Un, who is in his late 20s, in the wake of his father's death. Still, given his inexperience and age, there are questions outside North Korea about his leadership of a nation engaged in delicate negotiations over its nuclear program and grappling with decades of economic hardship and chronic food shortages.