China has demanded that a trilateral summit it was slated to attend in late May in Seoul with Japan and South Korea be postponed because of its ongoing dispute over the Japan-held Senkaku Islands in the East China Sea, sources said Wednesday.

South Korea, which is scheduled to chair the annual meeting of the three countries' leaders this year, has urged China to drop the demand, but Beijing has refused, the sources said, giving rise to speculation that the summit will not be held until June or later.

Beijing's demand comes despite signs of improved Sino-Japanese ties, which have soured over the Senkaku row, with former Prime Minister Yasuo Fukuda arranging to meet with Chinese President Xi Jinping in China later this month.

But the move suggests Beijing believes it is too soon for the leaders of China and Japan to meet face to face as a way to repair ties, amid continuing tensions over the Japan-controlled islets, which China also claims and calls Diaoyu.

The Chinese stance was conveyed to Japan via South Korea, according to the sources.

South Korea sounded out Japan and China about holding the trilateral summit in Seoul on May 25 and 26. But a Chinese official demanded that it be postponed, the sources said.

At a news conference in Tokyo late last month, Chinese Ambassador to Japan Cheng Yonghua took a cautious stance on a summit with Japan. "It would not be good for the top leaders to get into a fight as soon as they meet," he said.

The three-way summit has been held once a year since 2008, with the host country rotating every year.