TOKYO (Reuters) -- Japan has urged China to clarify whether it has banned ships from entering an area of the East China Sea and started work to expand a gas field in disputed waters, Japan's top government spokesman said on Monday.

Citing an unidentified website of the Chinese maritime authorities, Japanese media reports said China had issued a notice banning ships from the area while it laid pipelines and cables on the ocean floor as part of an expansion of the Pinghu gas field.

The work will last until Sept. 30 this year, the website said, according to the Japanese media reports.

Japan's Chief Cabinet Secretary Shinzo Abe said Tokyo had asked Beijing to make clear whether China had started to expand the gas field in the disputed waters. China's Foreign Ministry promised to reply as soon as possible, he said.

"Because the issue involves our sovereignty, we expressed our concern," Abe told reporters.

He said China's move could infringe on Japan's sovereignty and the UN convention on the Law of the Sea.

But he stopped short of saying that Tokyo would lodge a protest with China.

"We will continue to try to confirm the fact and make an appropriate response," he said.

Japan and China are involved in a stand-off over developing gas fields in the disputed area, one of a range of issues, including Japanese Prime Minister Junichiro Koizumi's visits to a Tokyo war shrine that China sees as a symbol of Tokyo's past militarism, that have hurt bilateral ties.

The two sides disagree over the position of the border between their exclusive economic zones in the East China Sea, and Japan fears that energy-hungry China's exploitation of the area could tap into resources in its own zone. "We have tried to resolve the issue through dialogue with a view supporting bilateral cooperation and joint development," Abe said. "There is no change in that stance." "It is important for the two countries to be future-oriented and have win-win talks."

Despite Japanese requests to halt development, China has continued work on its gas fields adjacent to waters over which Tokyo claims exclusive economic rights.

In response, Japan has granted test-drilling rights to Teikoku Oil Co., although drilling has not started.

Japanese Foreign Minister Taro Aso said last month that Tokyo might take counter-measures if China went ahead with full-fledged production at gas fields in the disputed areas.