Tokyo. US Secretary of State John Kerry has called on North Korea’s leaders to "come to the table in a responsible way” to end regional tensions.Speaking in Tokyo, the fourth and final stop on his Asian tour, Kerry warned North Korea it risked further isolation if its threats continued.He also reaffirmed the US commitment to defend its allies, including Japan.The countries he had visited were united in seeking the denuclearisation of the Korean peninsula, he said.Last week, Kerry visited South Korea, and at the weekend, he was in China, whose leaders he has said are "very serious” in their pledge to help reduce tensions - and to help end the North Korean nuclear programme.North Korea has recently threatened attacks against South Korea and the US, sparking alarm in the region.Speculation has been building that the North is preparing a missile launch, following reports that it has moved at least two Musudan ballistic missiles to its east coast.Japan is within range of these rockets and has been taking precautions, including setting up batteries of US-made Patriot anti-missile systems around the capital and sending two warships to the Sea of Japan, with orders to shoot down any missiles fired towards the Japanese islands.At a joint news conference with Foreign Minister Fumio Kishida, Kerry said North Korea had to understand by now "that its threats and provocations are only going to isolate it further and impoverish its people even further”."The US will do what is necessary to defend our allies against these provocations, but our choice is to negotiate,” he said.