New York - The United States on Monday urged the UN Security Council to impose the "strongest possible measures" against North Korea in response to its sixth and most powerful nuclear test."Only the strongest sanctions will enable us to resolve this problem through diplomacy," US Ambassador Nikki Haley told an emergency council meeting.Haley rejected as "insulting" a Chinese proposal for a freeze on North Korea's nuclear and missile programmes in exchange for a suspension of US-South Korean annual military drills."When a rogue regime has a nuclear weapon and an ICBM pointed at you, you do not take steps to lower your guard. No one would do that. We certainly won't," she declared.The United States, Britain, France, Japan and South Korea requested the urgent meeting after North Korea detonated what it described as a hydrogen bomb designed for a long-range missile.South Korea's defence ministry warned on Monday that Pyongyang may be preparing another missile launch after two tests in July of intercontinental ballistic missiles (ICBMs) that apparently brought the US mainland into range.Haley declared that the "time for half measures is over", suggesting the council must significantly ratchet up the pressure with biting sanctions to be decided in a new resolution.The US ambassador did not spell out what measures Washington would support, but diplomats have indicated that an oil embargo would have a crippling effect on the North Korean economy.Japan, France and Britain called for the swift adoption of a new sanctions resolution, but the call was expected to face opposition from Russia and China which maintain that sanctions alone will not resolve the crisis.Russia and China have called for diplomatic talks with North Korea to address the threat from its missile and nuclear tests.The council has imposed seven sets of sanctions on North Korea since it first tested a nuclear device in 2006, but Pyongyang has repeatedly found ways to circumvent the measures.The most recent resolutions, however, have significantly toughened the sanctions, targeting key exports sectors such as coal that are a source of hard currency for North Korean leader Kim Jong-Un's regime.