In a sign of Tokyo’s push to improve ties with Beijing, Japan plans to invite Chinese Premier Li Keqiang for a formal visit coinciding with a trilateral summit with South Korea in May, government sources said.

However, the invitation to South Korean President Moon Jae-in for the summit will likely be unofficial, possibly reflecting the chill in bilateral relations caused by renewed friction over the “comfort women,” Japan’s euphemism for the girls and women forced to provide sex for Imperial Japanese troops before and during World War II.

Japan has suggested to China and South Korea that the long-delayed trilateral summit take place in May, the sources said Friday. As years of tension between Japan and China begin to thaw, inviting Li for a formal visit is viewed as a step toward getting Chinese President Xi Jinping to eventually make a state visit. The last Chinese leader to make an official visit to Japan was then-Premier Wen Jiabao in May 2010.

Li is expected to be invited as an official working guest, a step down from state guest or official guest, but is likely to get a meeting with Emperor Akihito, according to the sources.

Ties between Japan and China have been strained in recent years by the sovereignty dispute over the Senkaku Islands, a group of uninhabited islets in the East China Sea administered by Japan but claimed by China and Taiwan as Diaoyu and Tiaoyutai, respectively.

China hardened its stance toward Japan after the government of Prime Minister Shinzo Abe’s predecessor, Yoshihiko Noda, purchased most of the islets from a private Japanese owner in 2012 to keep them out of the hands of former Tokyo Gov. Shintaro Ishihara, an outspoken nationalist who was mounting a separate bid to purchase the islets to formally bolster Japan’s ownership claim.

According to the sources, the government hopes to have Li visit for three days, including a formal summit and dinner with Abe. A Foreign Ministry source said Tokyo has suggested to Beijing that Li also visit somewhere outside the capital to develop a deeper understanding of Japan.

Abe’s national security adviser, Shotaro Yachi, may have brought up the plans in a meeting in Beijing last month with State Councilor Yang Jiechi, China’s top diplomat.

According to a source close to Sino-Japanese relations, Chinese Ambassador to Japan Cheng Yonghua said he was looking forward to Li’s visit, telling a senior member of Abe’s Liberal Democratic Party late last month that a formal visit “would hold great meaning for bilateral relations.” But the nature and duration of the stay is still not guaranteed. China has to first give the green light to the trilateral summit.

Japan’s overtures toward Li are in sharp contrast to its plans for Moon. Japan and South Korea are at odds over a historic 2015 agreement to settle the comfort women issue “finally and irreversibly.” But the Moon administration has been increasingly arguing that Japan needs to do more than stipulated by the 2015 agreement, which was negotiated and signed under his impeached predecessor Park Geun-hye.

The three countries have been rotating the trilateral summit-hosting duties since 2008, although the chill caused by the Senkaku issue put a stop to the gatherings in 2013 and 2014. After South Korea hosted the trilateral summit in 2015, Japan made plans to host in 2016 but canceled in light of the political turmoil enveloping Park.