International Atomic Energy Agency chief Yukiya Amano told Japan's top nuclear regulator in talks in Tokyo that it is crucial that the country share data with the international community about the safety of Japanese waters and marine life.
South Korea recently imposed a ban on fish from the area.
Japanese officials acknowledged in July that the Fukushima Dai-ichi plant has been leaking radioactive water into the Pacific Ocean since soon after its March 2011 meltdowns. Repeated leaks of contaminated water from storage tanks at the plant have added to the international concerns.
Tanaka said he hopes the joint monitoring will address concerns among Japan's neighbors and include experts from those countries.
Last month, South Korea banned all fish imports from Fukushima and seven other prefectures along Japan's northeastern coast, citing growing public worry over radioactive water leaks and insufficient information from Tokyo about the measures it has taken.
The plant's operator, Tokyo Electric Power Co, has come under fire over a spate of recent leaks and mishaps that have renewed doubts over its ability to handle the crisis. Experts have faulted TEPCO over delayed disclosures, human error and inadequate skills in measuring radioactivity, saying they have hurt the credibility of Japanese data.
Following criticism of its perceived reluctance to accept foreign expertise, Japan's government is stepping up efforts to allow international help. Prime Minister Shinzo Abe said on Sunday that Japan is open to foreign expertise in tackling the contaminated water problems at the plant.
