Peking's latest nuclear test, conducted on Saturday, has prompted an international outcry, despite China's new commitment to hold just one more test before September and then join an indefinite moratorium on nuclear testing.
The Greenpeace ship left Manila on Saturday and is carrying about 32 people from 12 countries. It is scheduled to approach Shanghai on Wednesday. The journey had been planned before the latest Chinese test and the timing was a coincidence, although it had been known that a test was due.
The Chinese Foreign Ministry has asked Greenpeace, which is about to open an office in Hong Kong, to abandon the mission, and the embassy in the Philippines refused permission to enter Shanghai.
Over the past week, China has apparently shown some signs of bowing to world pressure over its nuclear tests. Last week, in New York, it dropped its position that "peaceful" nuclear explosions be exempt from the Comprehensive Test Ban Treaty now under negotiation, and due to be signed in September at the United Nations General Assembly. On Saturday, it announced it would hold one more test before subscribing to the moratorium.
Since France completed its controversial tests in January, China has been the only country testing. Previously China had hinted it might not stop testing until the treaty was ratified.
Now, for the first time, China has announced its final test in advance, and analysts said Peking appeared to be trying not to upset the September signing. The Greenpeace ship poses a more immediate public relations challenge for China's government, which has to decide how to keep it out of Chinese waters; China's coastal patrols are not known for subtlety. Last August, China deported eight Greenpeace activists, including two photographers, over an anti-nuclear protest in Tiananmen Square.
While foreign ministries around the world at the weekend denounced Peking for its latest test, most analysts said that, if China sticks to its plan for one last blast, it will have carried out fewer than expected.
The latest explosion, conducted at the Lop Nor site in Xinjiang province, is China's 44th test, and China maintains it needs one more "to ensure the safety of its nuclear weapons". China has carried out far fewer tests than any other nuclear power.
Negotiations over the test ban treaty have to be completed by a 28 June deadline if it is to be ready for signing in September.