Prime Minister Shinzo Abe explained his decision to resign in separate calls with U.S. President Donald Trump and Russian President Vladimir Putin on Monday, while also touching on a number of issues involving the countries that remain unresolved.

During his 30-minute phone talks with Trump, Abe thanked him for helping to make the Japan-U.S. relationship "the strongest it has ever been," the Japanese government said.

Japanese Prime Minister Shinzo Abe (L) and U.S. President Donald Trump shake hands during their talks in New York on Sept. 25, 2019. (Kyodo)

It was Abe's first phone call with a foreign leader since he abruptly announced his resignation on Friday, citing a relapse of ulcerative colitis, an intestinal disease that can cause stomach pain and diarrhea. Abe just a week ago achieved the longest uninterrupted stint as prime minister in the country's history.

In a following teleconference, Abe and Putin reaffirmed their countries' commitment to continuing bilateral negotiations toward signing a postwar peace treaty, which has been hampered by a decades-old territorial row over Russian-held islands off Hokkaido.

Abe and Trump discussed North Korea's abduction of Japanese nationals in the 1970s and 1980s, the ongoing coronavirus pandemic and Japan's plans to renew its missile defense strategy, Chief Cabinet Secretary Yoshihide Suga said at a press conference.

Trump tweeted after the call that Abe is a "special man" who will "soon be recognized as the greatest Prime Minister in the history of Japan, whose relationship with the USA is the best it has ever been."

Japanese Prime Minister Shinzo Abe (L) and Russian President Vladimir Putin shake hands at an economic forum in Vladivostok, Russia, on Sept. 5, 2019. (Kyodo)

The two men built a rapport since Trump came to power in 2017 and have played several rounds of golf together. "The two leaders said they look forward to continuing their wonderful friendship for years to come!" the White House said in a statement.

Abe will remain in office until his successor is chosen. The ruling Liberal Democratic Party, led by Abe, is considering holding a leadership election on Sept. 14, party lawmakers said.

The prime minister assured Trump whoever succeeds him will be committed to strengthening the Japan-U.S. alliance, according to a Japanese official.

The abduction issue has been one of Abe's main focuses, though he has struggled to make progress toward a resolution despite Trump raising the matter in summits with North Korean leader Kim Jong Un.
Abe asked Trump for continued U.S. support on Japan's efforts to resolve the issue with North Korea, the official said.

The two leaders also agreed that their nations will cooperate on the development of a vaccine and treatment for the novel coronavirus that has ravaged the global economy. The pandemic has led to the deaths of more than 845,000 people worldwide, according to Johns Hopkins University.

Meanwhile, Putin praised Abe for making great contributions to removing obstacles between the two countries, according to the Japanese government.

Abe explained his resignation and said he hopes active talks will be held in the future toward resolving the territorial issue.

At the end of the 20-minute teleconference, Putin said "arigato," thanking Abe in Japanese, while Abe reciprocated in Russian, "Vladimir, spasiba," according to the government.

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