Tokyo - North Korea says it will make the United States pay a heavy price if a proposal Washington is backing to impose the toughest sanctions ever on Pyongyang is approved by the UN Security Council this week.The North's Foreign Ministry issued a statement early on Monday saying it is watching the United States' moves closely and threatened it is "ready and willing" to respond with measures of its own.The United States has called for a vote on Monday, New York time, on new UN sanctions against North Korea.Last Tuesday, the US circulated a draft resolution proposing the toughest UN sanctions on North Korea to date, including a ban on all oil and natural gas exports to the country and a freeze on all foreign financial assets of the government and its leader, Kim Jong Un.Security Council diplomats, who weren't authorised to speak publicly because talks have been private, said the US and China were still negotiating the text late on Sunday.Previous UN sanctions resolutions have been negotiated between the United States and China, and have taken weeks or months. But the Trump administration is demanding a vote in six days.Nuclear test North Korea conducted its sixth nuclear test a week ago and has been launching ballistic missiles at a record pace. Both are violations of UN resolutions, but Pyongyang claims it must carry them out to build nuclear deterrent against what it sees as US aggression.According to a recent study by the Nautilus Institute think tank, a massive cutback in the flow of oil from China would definitely hurt the North Korean economy, and especially average citizens. But the report said the impact would likely be blunted on the military, which probably has enough fuel stockpiled to continue normal operations for the immediate future.Russian President Vladimir Putin recently expressed doubt over whether sanctions are an effective means of getting the North to stop its missile and nuclear testing, and China, harbouring similar concerns, has repeatedly hesitated in the past to fully support US sanction plans.German Chancellor Angela Merkel on Sunday also stressed the importance of diplomacy and offered to act as a facilitator if needed.The five permanent members of the UN Security Council and Germany conducted long-running talks with Iran that led to a 2015 deal for international sanctions to be lifted in exchange for Tehran curbing its nuclear activities."I could also imagine such a format to settle the North Korea conflict," she said.