Bloomberg said on NBC's "Meet the Press" gun violence was out of control in the United States with a body count that was multiplying at a wartime pace.
"If he (Obama) does nothing during his second term, something like 48,000 Americans will be killed with illegal guns," Bloomberg said. "That is roughly the number of Americans killed in the whole Vietnam War."
Bloomberg said shooting sprees like the one that claimed 28 lives in Newtown, Conn., Friday were becoming increasingly frequent and could no longer be downplayed as isolated incidents. He said Obama needed to take control of the issue and seek tighter controls on firearms even if he is rebuffed by Congress.
"He at least has to try," said Bloomberg, who as long advocated stricter rules on guns. "That's his job."
The mayor also expressed impatience with Obama, whom he saw as overly timid in the face of opposition from gun-rights organizations.
"The president campaigned in 2008 on an assault weapon ban, and the only gun legislation that the president has signed since then is the right to carry a gun in national parks where our kids play and to carry guns on Amtrak. I assume that's to stop the rash of train robberies which stopped back in the 1800s."