Japan's Prime Minister Shinzo Abe, who has unexpectedly announced his resignation, plans to hold phone talks with U.S. President Donald Trump on Monday and call for bilateral cooperation to continue under his successor, Japanese and U.S. government sources said Saturday.

Abe, who said Friday he will step down from the post he has held since 2012, is expected to tell Trump that he had to resign before the end of his term to undergo prolonged treatment for his ulcerative colitis, a chronic illness.

The sudden resignation announcement came just a few days after Abe achieved the longest uninterrupted stint as premier in the country's history. Since the announcement, the race to find Abe's successor has been intensifying.

U.S. President Donald Trump (R) claps, alongside Japanese Prime Minister Shinzo Abe, as they watch the Summer Grand Sumo Tournament at Ryogoku Kokugikan in Tokyo on May 26, 2019. (Kyodo)

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During their phone conversation, Abe is expected to ask Trump, with whom he has established a close relationship, for the United States to continue its strong bilateral cooperation with the new Japanese leadership, according to the sources.

Abe may also discuss Japan's missile defense policy after his government in June scrapped a plan to deploy a U.S. missile defense system known as Aegis Ashore, the sources said.

U.S. President Donald Trump said Friday he pays his "highest respect" to Abe, expressing sympathy that "a great friend" had to decide to step down due to health problems.

Abe and Trump held phone talks in May when they agreed to cooperate over their countries' coronavirus response and development of treatment drugs and vaccines.

During Friday's press conference, Abe said he will remain in office until his replacement is selected. The ruling Liberal Democratic Party, led by Abe, is considering holding a leadership election on or around Sept. 15, according to senior party lawmakers.

The prime minister's health came under fresh scrutiny with his repeated hospital visits after he kept a low profile without holding any press conferences for nearly 50 days through early August, despite calls for him to explain the government's handling of the coronavirus to the public.

The resignation was a flashback to 2007, when the illness forced Abe to resign, only a year after becoming the country's youngest premier in the postwar era at age 52.