Hong Kong has started strengthening radiation inspections of seafood imports from Japan, sources said Saturday, amid concerns in China over a plan to discharge treated radioactive water from the crippled Fukushima nuclear plant into the ocean.

The tighter inspections started in mid-June and customs clearance of Japanese seafood was delayed by about three hours compared with earlier clearance times.

Hong Kong on Friday reiterated its concerns over the water plan. The Japanese government has confirmed that some shipments of Japanese seafood have been held up at China's customs.

Hong Kong is Japan’s second-largest market for agricultural and fisheries exports, while mainland China is its biggest.

Japan’s plan, approved by U.N. nuclear watchdog the International Atomic Energy Agency (IAEA), has faced opposition at home and abroad over concerns for food safety. Tokyo says the releases will be safe and meet global standards.

China has been among the most vocal in its opposition to the plan, accusing Japan of treating the Pacific Ocean as its “private sewer.”

Hong Kong's current ban on shipments from one prefecture would “definitely” be expanded, Chief Executive John Lee told a media briefing earlier this month.

“If the exercise really starts, we’ll be banning a large number of prefectures’ sea products," he said.

Foreign Minister Yoshimasa Hayashi, who has accused China of disseminating “incorrect information,” has urged Beijing to deal with the matter “from a scientific standpoint.” After talks with China's top diplomat last week, Hayashi said he also conveyed to Beijing his objection to “politicizing” the issue.