South Korea's Yonhap news agency says North Korea appears to be preparing for a third nuclear weapons test.

The report quotes an unidentified intelligence source as saying satellite images show workers digging a new tunnel at the Punggye-ri nuclear test site in the northeast. The North conducted two previous nuclear tests in tunnels at the site — first in October of 2006 and again in May of 2009.

The source told Yonhap it appears the tunnel work is in its final stage.

North Korea agreed in February to stop nuclear tests, uranium enrichment and long-range missile launches. Days later, U.S. Secretary of State Hillary Clinton announced U.S. plans to provide the impoverished North with more food aid.

The agreement quickly unraveled when Pyongyang announced plans to launch a long-range missile this month, in defiance of United Nations resolutions. Western journalists allowed to visit the launch site said Sunday the missile has been installed on the platform.

Pyongyang says the planned launch is aimed at placing a weather satellite into space. But other countries — including the United States, Japan and South Korea — view it as a disguised missile test.

North Korea has said the launch — set to occur between April 12 and April 16 — is to commemorate the 100th birthday of its late founding leader, Kim Il Sung.

China's Foreign Minister Yang Jiechi Sunday said Beijing is troubled by the planned launch. The official Xinhua news agency also quotes him as urging more international diplomacy to resolve the situation.

The United States, China, Russia, South Korea and Japan have jointly negotiated with the North to end its nuclear program. But those talks broke down in late 2009, when Pyongyang expelled international inspectors and then conducted its second nuclear test.