Washington – Despite the US warning on Sunday of a "massive military response" to any threat from North Korea, the Trump administration has few good options to force the North to rein in its nuclear and missile programmes following Pyongyang's most powerful nuclear test yet.Its best hope may be to further expand its already wide-reaching economic sanctions against the North, hoping this new pain might finally bring Kim Jong-Un to show restraint.A military strike? UnlikelyNorth Korea's latest nuclear test does not seem to have altered the American equation, though it may have toughened US rhetoric. Defence Secretary Jim Mattis said on Sunday that "any threat to the United States or its territories, including Guam or our allies, will be met with a massive military response".And President Donald Trump tweeted earlier of the North Koreans that "they only understand one thing" – presumably force.But experts said force has clear limits."There are no realistic military options in terms of striking North Korea, because doing so would likely spark a full-scale war," Mark Fitzpatrick, executive director for America of the International Institute for Strategic Studies, told AFP.The North has massed powerful artillery units at the border of South Korea capable of wreaking immense destruction on Seoul, a city of 10 million just 55km away. An American strike against the North could thus spark a conflagration between the two Koreas that could spread quickly into a regional conflict."Before everyone goes nuts, a nuclear test by North Korea is a troubling development but does not change the nature of the challenge we face," tweeted Jon Wolfsthal, of the Carnegie Endowment for International Peace."Such a test does not require a military response, which is good because we don't have any viable options."Applying military pressureWithout actually striking, the United States can increase its military pressure on Pyongyang. Before the North's latest nuclear test, the American and South Korean presidents had agreed to strengthen Seoul's missile capabilities – a way to bolster its dissuasive capacity."Viable military options include moves such as deploying additional assets to the region," Fitzpatrick said. "Note that South Korea now wants to consider redeploying US tactical nuclear weapons" – a move he called complicated but viable.The US military withdrew all its tactical arms from South Korea 25 years ago.Another form of pressure requires no weapons – the sort of bellicose rhetoric Trump employed when he uttered his famous phrase about unleashing "fire and fury" on the North. On Sunday, Mattis even evoked the possibility of the "total annihilation" of the North, should Pyongyang bring matters to a head.But rhetoric, too, has its limits. During a lull late last month after Trump's "fire and fury" comment, the US president said he thought Kim was "starting to respect us", but days later the North sent a missile sailing over Japan.And on Sunday came the test of a bomb more powerful than the one that destroyed Hiroshima.Again, economic sanctionsThe American administration seemed on Sunday to be leaning toward economic sanctions."The United States is considering, in addition to other options, stopping all trade with any country doing business with North Korea," Trump tweeted. The United States is considering, in addition to other options, stopping all trade with any country doing business with North Korea.— Donald J. Trump (@realDonaldTrump) September 3, 2017 Treasury Secretary Steven Mnuchin said he would propose a series of tough economic sanctions for Trump's consideration aimed at any country doing business with the North. He said he wanted to work with US allies and with China, which buys 90% of all North Korean exports.On August 22, the US announced sanctions against six individuals and 10 companies from Russia and China for doing business with the North.The United States was behind the last set of United Nations sanctions against North Korea, adopted unanimously on August 6 by the Security Council, with the notable support of China and Russia. That seventh series of sanctions aimed to deprive the North of a billion dollars in revenues from sales of coal, iron and seafood.The next step from the UN could be a full or partial petroleum embargo.Another option mentioned by British officials at the UN: sanctions to require Russia and China to expel North Korean expatriate workers – an important revenue source for Pyongyang.