China told the UN General Assembly it was worried about radioactive water leaks from the Japanese plant, which went into meltdown after being hit by a tsunami in March 2011.
"China follows closely the countermeasures to be adopted by Japan," China's deputy UN ambassador Wang Min told a debate on the International Atomic Energy Agency (IAEA).
"We urge the Japanese side to spare no effort in minimising the subsequent impact of the accident and provide timely, comprehensive and accurate information to the international community," Wang added.
China and Japan have a series of bilateral disputes, and the new comments are certain to annoy the Japanese government, which is already under major domestic pressure over Fukushima, diplomats in New York said.
Wang said the 2011 disaster had "sounded the alarm bell for nuclear safety" even though China "firmly" supports the use of nuclear power.
South Korea also said it was worried about the radioactive leaks but gave more support to Japan.
Fukushima "continues to be a source of serious concern, especially to adjacent countries, because of the spillage of contaminated water into the sea," said South Korea's deputy UN ambassador Sul Kyung-Hoon.
South Korea "appreciates the Japanese government's efforts to share relevant information with the international community," Sul added, while calling on the IAEA to strengthen assistance to Japan.
IAEA director general Yukiya Amano said the UN atomic watchdog "has recommended that Japan establish an effective plan and mechanisms for the long-term management of contaminated water.
"The announcement by the Japanese government of a basic policy for addressing this issue was an important step forward," Amano added.
The first batch of IAEA experts arrived in Japan on Wednesday at the invitation of the Japanese government as it looks to bolster its credibility.
The two researchers from the Environment Laboratories in Monaco are planning to analyse sea water near Fukushima, the agency said.
Their analysis will contribute to the IAEA-led international peer review of Tokyo's roadmap towards decommissioning the destroyed reactors, it said.
"One of the focuses of the mission is the contaminated water issue," the agency said.
The experts are David Osborn, director of Environment Laboratories in Monaco, and Hartmut Nies, head of the Radiometrics Laboratory, it said.