WASHINGTON (AP) — President Donald Trump said he has accepted an invitation from Chinese leader Xi Jinping to visit Beijing in April and that he reciprocated by inviting Xi for a state visit to the U.S. later next year.

Trump made the announcement a few hours after he spoke with Xi on the phone on Monday morning, in which he said the two men discussed issues including Ukraine, fentanyl, and soybeans. The phone call came nearly one month after the two men met in person in the South Korean city of Busan.

“Our relationship with China is extremely strong!” Trump said.

Beijing, which announced the phone call first, said nothing about the state visits but said that the two leaders discussed trade, Taiwan and Ukraine.

Xi told Trump in the phone call Monday that Taiwan’s return to mainland China is “an integral part of the post-war international order,” and he expressed hope for a “fair, lasting and binding peace agreement” over Ukraine, according to the Chinese foreign ministry.

The conversation came after Japanese Prime Minister Sanae Takaichi recently said Japan’s military could get involved if China were to take action against Taiwan, the self-governing island that Beijing says must come under its rule. Japan is an important ally of the U.S. in the region. The phone call also coincided with the latest push by the Trump administration to end the war in Ukraine.

The Chinese, who in the past always pointed out that their leader picked up the call “upon request”, didn’t say such for Monday’s call. “That means China called Trump,” said Sun Yun, director of the China program at the Washington-based think tank Stimson Center

“My best guest is China is worried about the escalation (in tensions) with Japan. The reference to Taiwan and the post-WWII order directly points to the spat with Japan over Taiwan,” said Sun Yun, director of the China program at the Washington-based think tank Stimson Center. “They also talked about Ukraine. That is an issue China is interested in due to the new peace negotiation.”

China's relations with Japan sour

China-Japan relations have plunged to a new low following Takaichi's remarks, with Beijing denouncing her words. Over the weekend, Chinese Foreign Minister Wang Yi said Japan “crossed a red line that should not have been touched.”

Xi in the phone call said China and the U.S., which fought together during the war against fascism and militarism, should “jointly safeguard the victory of World War II.” The U.S. has taken no side on the sovereignty of the self-governed island but is opposed to the use of force to seize Taiwan. It is obligated by a domestic law to provide sufficient hardware to the island to deter any armed attack.

Trump has maintained strategic ambiguity about whether he would send U.S. troops in case of a war in the Taiwan Strait. His administration has urged Taiwan to increase its defense budget.

Earlier this month, Taiwan's foreign ministry said it received official notification that the Trump administration approved a US$330 million arms sales to Taiwan, including fighter jet parts. Beijing immediately protested the arms sale, saying it “grossly violated” the one-China principle, by which Beijing considers Taiwan to be part of Chinese territory. “China deplores and opposes that,” Chinese foreign ministry spokesperson Lin Jian said then.

The two leaders also discussed the Ukraine crisis, the Chinese side said, with Xi saying the crisis should be resolved “at its root.” The Chinese leader stressed Beijing's support for “all efforts that are conducive to peace,” according to the statement. However, western governments have accused Beijing of enabling the war through its industrial support for Moscow.

Trump and Xi discussed trade

Trump said he spoke with Xi about “Fentanyl, Soybeans and other Farm Products, etc.”

“We have done a good, and very important, deal for our Great Farmers — and it will only get better,” Trump wrote.

Since he met Xi in Busan, “there has been significant progress on both sides in keeping our agreements current and accurate,” Trump said.

In the call, Xi said the bilateral relationship has “generally maintained a steady and positive trajectory” following the Busan summit, and he said the two sides should strive to make “more positive progress,” according to the Chinese foreign ministry. It didn't reveal any concrete agreements on matters such as purchases of American soybeans.