Seoul - Tensions between North Korea and the United States are spiking again in the aftermath of Pyongyang's latest weapons test, when it lobbed an intermediate range missile over northern Japan.The long-running face off between the little-understood regime in Pyongyang and the United States has intensified since the start of Donald Trump's unorthodox presidency, and shows no signs of calming down.Here are some questions and answers on the volatile situation:What has happened?In the last two months, Pyongyang has carried out its first two successful tests of an intercontinental ballistic missile (ICBM) apparently bringing much of the US mainland into range.It has also threatened to send a salvo of missiles to "envelop" the US Pacific territory of Guam, which is home to significant US military facilities and which it sees as a beachhead for invasion.US President Donald Trump has issued apocalyptic warnings of raining "fire and fury" on the North, saying Washington's weapons were "locked and loaded", while his administration has repeatedly said military action was an "option on the table".Tuesday's firing of a missile over Japan - which Pyongyang called a "curtain-raiser" for further launches - was seen as an escalation by the regime because of its trajectory.The North says it needs nuclear weapons to defend itself against invasion by the United States, and its possession of an ICBM enhances its deterrent, by enabling it to threaten US cities as well as South Korea and Japan, both of them US allies. What will the North do next?Leader Kim Jong-Un has called for Pyongyang to carry out more missile tests into the Pacific Ocean, and there is no reason not to take him at his word.Pyongyang has a long history of steadily pushing back boundaries.Last year it fired a missile that came down in Tokyo's exclusive economic zone - waters extending 200 nautical miles from Japan's coast. At the time it provoked consternation and condemnation, but there were several repetitions in the following months.Analysts say the North's next escalation could be to fire multiple missiles over Japan simultaneously. It could also look to demonstrate its capabilities by sending one further than the 2 700km distance Tuesday's missile travelled - Guam is around 3 400km from the North.But the flight path of Tuesday's missile appears to have been carefully chosen not to go anywhere near Guam and risk provoking a physical response -- KCNA went as far as declaring: "The drill had no impact on the security of the neighbouring countries."Is US military intervention possible?Experts caution that military intervention in North Korea remains unlikely -- at least for now.Still, the Pentagon has 28,500 troops in South Korea and detailed plans for a potential conflict with the North. It has spent decades rehearsing with South Korean counterparts, including in the annual Ulchi Freedom Guardian joint exercises which were ending Thursday.Options range from limited surgical strikes on nuclear targets to a pre-emptive "decapitation" attack to take out Kim and other senior leaders.But North Korea also has decades of experience in tunnelling - it dug passages under the Demilitarized Zone (DMZ) after the Korean War - and both its atomic arsenal and conventional artillery are believed to be well underground, protected from attack.A first strike would be highly unlikely to destroy all its nuclear weapons, and it has massed artillery within range of Seoul, home to 10 million people just 55 kilometres (30 miles) from the DMZ - giving it the ability to inflict mass casualties even with just a conventional response.Despite Trump's rhetoric, many agree with his former chief adviser Steve Bannon, who told The American Prospect: "There's no military solution, forget it."What about China and more sanctions?In August the UN Security Council passed a seventh set of sanctions against the North over its weapons programmes, including bans on the export of coal, iron and iron ore, lead and lead ore as well as fish and seafood.The measures were approved unanimously - including by Moscow and Beijing, the North's sole major ally, but their effectiveness hinges largely on China, which accounts for 90% of trade with North Korea but is suspected of failing to enforce past UN measures.The US has repeatedly tried to press China into taking a harder line on North Korea, but Beijing fears a collapse of Kim's regime.Trump and Japanese Prime Minister Shinzo Abe agreed this week to increase pressure on the North.That could also include unilateral moves by Japan and the US - Washington last week sanctioned 16 Chinese and Russian individuals and companies, accusing them of supporting the North's nuclear programme.Beijing and Moscow both back a mutual freeze in which the North stops missile tests and the US and South Korea halt joint military exercises.But that would leave the North with capabilities seen as unacceptable by Washington, which says the proposal would reward bad behaviour. How about talks?The message out of Washington is a little muddied.After the latest launch, Trump tweeted: "The US has been talking to North Korea, and paying them extortion money, for 25 years. Talking is not the answer!" But soon afterwards Defense Secretary Jim Mattis told reporters: "We're never out of diplomatic solutions," and Secretary of State Rex Tillerson has repeatedly said he hopes to persuade Pyongyang to come to the negotiating table.Pyongyang insists its weapons are not up for negotiation, and analysts say it is enhancing its military capabilities to strengthen its position in any future talks with the US - it has shown no interest in discussions with Seoul.Through the 2000s, six-party talks among China, Russia, Japan, South Korea, North Korea and the US appeared to draw Pyongyang, then under the rule of Kim's father Kim Jong-Il, towards a possible slowdown in its programme, but that process collapsed in 2009. The North has carried out five nuclear tests, two of them last year, and says it has mastered both miniaturisation and re-entry technology to fit a working warhead onto a missile.Questions remain about those and other issues, but Professor Koh Yu-Hwan at Dongguk University in Seoul told AFP that this week's launch was "a clear message" the North would not talk until it had nuclear-tipped missiles that could reach the US.Having such a weapon, Pyongyang believes "will give the North the upper hand in any negotiations", he said.