In a landmark telephone conversation, Prime Minister Shinzo Abe and Chinese President Xi Jinping on Friday agreed to closely cooperate in resolving the North Korean nuclear and missile issues and confirmed the importance of U.N. sanctions against the North to achieve that goal.

In the first telephone talks ever between a Japanese prime minister and Chinese president — demonstrating the rapidly warming ties between Asia’s two biggest powers — Abe and Xi welcomed the recent commitment by North Korean leader Kim Jong Un and South Korean President Moon Jae-in toward a “complete” denuclearization of the Korean Peninsula.

The two leaders also agreed to cooperate on resolving the abduction issue, a senior government official said.

Xi’s promise to help resolve the long-stalled issue, which involves Japanese abducted in the 1970s and 1980s, is positive news for Abe amid concerns Japan is being left behind by the five-way surge in diplomatic activity. Abe has said the issue is one of his top priorities.

In Washington the same day, the abductees’ kin called for support as U.S. President Donald Trump prepares to meet with Kim.

The request was made at a meeting with Matt Pottinger, senior director for Asian affairs at the U.S. National Security Council. The U.S. fully understands that all the abductees must be brought home, Pottinger said, adding Washington will work on the issue.

The family members who met with Pottinger included Takuya Yokota, a younger brother of Megumi Yokota, who was kidnapped to North Korea in 1977 at the age of 13, and Koichiro Iizuka, the eldest son of Yaeko Taguchi, who was abducted in 1978 when she was 22 years old.

They handed Pottinger handwritten messages saying, “Return my beloved sister” and “I want to see my mother. Give her back.”

Takuya Yokota said that he wants the messages to be delivered to the North Korean regime if the opportunity arises.

The day’s meeting was joined by Katsunobu Kato, Japanese minister in charge of the abduction issue.

The phone call between Abe and Xi was held at Japan’s request, with Tokyo eager to win Xi’s cooperation on North Korea issues.

China is North Korea’s closest ally. Xi met with Kim on March 26, ahead of the historic inter-Korean summit on April 27.

Chinese broadcaster CCTV said Xi told Abe he hopes Japan will play constructive roles in the saga, saying he wants to resolve the North Korean issues through dialogue and in a balanced manner by taking into account the interests of the related countries.

To ensure the enforcement of U.N. sanctions on the North, Abe and Xi confirmed they will also address its attempts to evade them via ship-to-ship transfers and other means, the official said.

Japan has noted and reported multiple cases of suspected transfers to the United Nations involving North Korean and foreign-registered tankers in the East China Sea since January.

Abe also expressed a desire to visit China to reciprocate for an earlier visit by Xi. On Friday, they agreed to further deepen ties recently strained by territorial and historical issues.

“We agreed to heighten Japan-China relations … to the next stage by significantly expanding public exchanges in various areas” with this year marking the 40th anniversary of the signing of a friendship treaty between the two countries, Abe told reporters.

Abe underscored the significance of the talks, telling the Chinese president, “It’s productive that I can directly exchange views with (you) at a time when the situation over North Korea is evolving greatly,” a Japanese official said.

Xi also told Abe the talks are a demonstration of the forward-looking change in the bilateral relationship, the official said.

The 40-minute chat came days before a trilateral summit involving Japan, China and South Korea on Wednesday in Tokyo, and as Trump and Kim prepare to meet in the coming weeks for the first U.S.-North Korea summit.

The trilateral summit is to be attended by Abe, Moon and Chinese Premier Li Keqiang. Abe is expected to hold bilateral talks with Li and Moon on the side.A senior Chinese diplomat said North Korea is not going to be a focus of the summit.

Chinese Vice Foreign Minister Kong Xuanyou said Li’s trip to Japan, the first by a Chinese premier in eight years, represented a “rare development opportunity,” though he admitted challenges remain.

“There will be no off-limits areas,” Kong told reporters, referring to the talks between Li and Abe.

“As long as there are subjects both are interested in, they can be put on the table for candid discussion. (We) hope to increase understanding through discussion, which is helpful to narrowing differences on certain problems.”

While North Korea and other regional issues would be discussed with both Japan and South Korea so the three can better coordinate, the isolated country is not likely to be a focus of the meeting.

“I personally think this China-Japan-South Korea meeting is not to mostly discuss the situation on the Korean Peninsula. It’s mainly to discuss regional cooperation by the three,” said Kong, who is also China’s special envoy for the North Korean nuclear issue.

“So I think it will be hard for the three of them to have sufficient time to have deep talks on this issue.”

Li will travel to Indonesia before going to Japan.

Indonesia is seeking ways to speed up a $5 billion high-speed rail project being built by a consortium of local and Chinese state firms that faces obstacles over land ownership issues.