TOKYO - A US military plane capable of detecting radiation has taken off from southern Japan amid concerns over a threatened nuclear test by North Korea, a news report said on Thursday. The plane, which can collect and analyze radioactive substances in the air, took off from the US air base at Kadena on the southern island of Okinawa, Kyodo News agency reported.The plane is believed to be monitoring signs of a possible nuclear test by North Korea, Kyodo said. Col. Anne Morris of US Forces Japan refused to confirm the Kyodo report, saying she was not authorized to talk specifically about ongoing operations, but said US forces were on alert for any moves by the North. “Of course, everybody is being vigilant. But the US position remains that we hope North Korea will allow the situation to be resolved diplomatically,” she said.