AFP

US SECRETARY of State Hillary Clinton says the US cannot rule out using nuclear weapons if it came under biological attack, saying in that case "all bets are off".

"If we can prove that a biological attack originated in a country that attacked us, then all bets are off," Ms Clinton said in an interview with CBS's Face the Nation.

Ms Clinton was referring to a new US nuclear policy unveiled last week that restricted the use of atomic weapons against non-nuclear states that comply with the Nuclear Non-Proliferation Treaty (NPT).

Both Ms Clinton and Defence Secretary Robert Gates said in joint television interviews that Iran and North Korea represent exceptions to the limits on a US nuclear response, as both have defied UN resolutions on their atomic programs.

"We leave ourselves a lot of room for contingencies," she said.

Asked why Iran and North Korea are considered exceptions, Mr Gates said: "Well, because they're not in compliance with the nuclear non-proliferation treaty. So for them all bets are off. All the options are on table."

Ms Clinton and Mr Gates said a new arms control deal with Russia and the revised nuclear policy will bolster President Barack Obama's diplomatic efforts to isolate Iran and North Korea over their nuclear programs.