SEOUL/TOKYO - The United States has deployed missile defense systems that will work with the Japanese and South Korean militaries to track a rocket that North Korea says it will launch some time over an 18-day period beginning on Monday.North Korea has notified UN agencies that it would launch a rocket carrying what it called an earth observation satellite some time between Feb. 8 and Feb. 25, triggering international opposition from some governments that see it as a long-range missile test.North Korea says it has a sovereign right to pursue a space program.Coming so soon after North Korea's fourth nuclear test, on Jan. 6, a rocket launch will raise concern that it plans to fit nuclear warheads on its missiles, giving it the capability to strike South Korea, Japan and possibly the US West Coast.China has told North Korea that it does not want to see anything happen that could further raise tension, Chinese Foreign Minister Wang Yi said, describing "a serious situation", after a special envoy from China visited North Korea this week.China is the North's sole major ally but it disapproves of its nuclear program. The United States has urged China to use its influence to rein in its neighbor.A launch would draw fresh US calls for tougher UN sanctions that are already under discussion in response to the nuclear test.What would likely be an indigenous three-stage rocket will be tracked closely. South Korea and Japan have put their militaries on standby to shoot down the rocket, or its parts, if they go off course and threaten to crash onto their territory."We will, as we always do, watch carefully if there's a launch, track the launch, (and) have our missile defense assets positioned and ready," US Defense Secretary Ashton Carter said on Thursday."We plan a lot about it. We and our close allies - the Japanese and the South Koreans - are ready for it."