WASHINGTON—World leaders have found a new route to get a read on official U.S. thinking: straight to the top.

Increasingly, savvy leaders are bypassing the standard protocols and government processes of American diplomacy to go directly to President Trump himself, according to current and former officials, allies and foreign-policy experts.

North Korea’s Kim Jong Un, Turkey’s Recep Tayyip Erdogan and Russia’s Vladimir Putin are among the heads of state who have cut out the middle layers of aides and agency officials to talk to Mr. Trump. For many, it’s the most direct approach to the president’s sometimes unpredictable policy decisions.

Mr. Trump has encouraged such approaches, but they have added a layer of ambiguity to foreign affairs. Some aides fret that the personal talks can sow confusion within the administration. At times, senior officials have been left in the dark or had to backtrack on some of Mr. Trump’s remarks.

Mr. Trump with North Korean leader Kim Jong Un on Feb. 28 in Hanoi. Photo: Evan Vucci/Associated Press

Supporters point to Mr. Trump’s hands-on strategy for making deals, drawn from his business background, as one of his strengths. He has given out his personal cellphone number to allies including Canada’s Prime Minister Justin Trudeau and France’s President Emmanuel Macron, said U.S. and foreign officials, so they can speak freely without the bureaucratic headaches and security measures that often come with official calls.

“At this point, foreign leaders understand that nobody can speak authoritatively other than Trump, and that what other interlocutors say may not represent the president’s position today,” said Robert Danin, a longtime U.S. diplomat, now at the Council on Foreign Relations and the Harvard Kennedy School’s Belfer Center.

The White House didn’t respond to requests for comment.

Mr. Trump has extolled his “very strong and personal relationship” with China’s President Xi Jinping on Twitter and elsewhere, adding in a tweet in December, “He and I are the only two people that can bring about massive and very positive change, on trade and far beyond, between our two great Nations.”

After February’s failed summit with North Korea, however, delicate U.S.-China trade talks have taken a more conventional turn over concern on both sides, particularly China’s, that a meeting of the two top leaders could go awry. Now, top aides are toiling away at details before a potential meeting in Florida between the two leaders later this month.

Mr. Trump has opted to hold meetings with Mr. Putin without standard note-taking, out of concerns about previous leaks of his conversations with foreign leaders. Such notes are typically circulated among government agencies to keep everyone up-to-date on the latest discussions.

Some advisers worry that Mr. Trump, a foreign-policy novice when he became president, could overpromise where the U.S. isn’t prepared to deliver, or fail to properly hold leaders accountable on issues with national security implications.

After a July 2018 meeting with Mr. Putin in Helsinki, Mr. Trump questioned the U.S. intelligence conclusion that Moscow had meddled in the 2016 U.S. election. “President Putin was extremely strong and powerful in his denial,” he said at a news conference. Mr. Trump reversed his comments the next day, saying he supported U.S. intelligence agencies.

An official familiar with the inner workings of the White House said the president’s advisers suspect that he regularly speaks with world leaders on his phone.

“We never know who he’s talking to or what he’s agreed to,” the official said.

Mr. Trump and Russian President Vladimir Putin last year in Helsinki. Photo: brendan smialowski/Agence France-Presse/Getty Images

Tensions between the U.S. and Turkey were simmering when Mr. Erdogan picked up the phone and called Mr. Trump in January.

Mr. Trump’s national security adviser, John Bolton, had clashed with Turkish officials over U.S. plans to withdraw from Syria. Mr. Erdogan wanted to hear about those issues from Mr. Trump himself, “bypassing everyone else” by calling the president directly, a U.S. official said.

A Turkish official said, “There was a sense that it is better to wait and see what President Trump will say on the matter before any actions are taken.”

Mr. Trump has said that he’s his own best spokesman and has criticized his own administration’s ability, at times, to portray a unified public message.

“It is not clear Trump really wants others speaking for him unless it is to quote verbatim what he has already said,” said the Council on Foreign Relations’s Mr. Danin.

Mr. Trump’s supporters applaud his “Art of the Deal” approach to diplomacy. Mr. Trump has repeatedly said that he would be willing to meet anyone, including the leaders of North Korea and Iran, if he felt that he could secure some sort of agreement or an assurance from the other side that an agreement is possible.

Mr. Trump’s preference for one-on-one discussions is his way of cutting through the red tape that often dictates international negotiations, according to those familiar with his thinking.

Mr. Trump walked with Chinese President Xi Jinping at Mar-a-Lago in West Palm Beach, Fla., in 2017. Photo: jim watson/Agence France-Presse/Getty Images

Before his first meeting with North Korea’s Mr. Kim last year, Mr. Trump told reporters he would know “within the first minute” whether talks would succeed. “Just my touch, my feel.That’s what I do.”

Mr. Kim has shunned some of Mr. Trump’s most senior diplomats, including Secretary of State Mike Pompeo, and sent letters directly to Mr. Trump suggesting a personal bond.

In one letter, Mr. Kim noted that only they could address the longstanding conflict between the two countries, according to several people briefed on the contents of the letter.

“Kim Jong Un has concluded that Trump is a good guy to talk to, while conversations at a lower level are a waste of time,” said Wi Sung-lac, a former South Korean diplomat.

Hopes ran high around last month’s summit between the U.S. and North Korea in Hanoi, with the White House announcing one day before the formal meeting that a joint agreement would be signed the next day. The meeting ended early without one.

Mr. Trump’s stance on policy has caught his own aides off guard. His December order to withdraw troops from Syria abruptly shifted U.S. policy and drew concern from within the White House and Congress, prompting his administration to slowly walk it back and convince Mr. Trump to leave a few hundred troops in the country.

In June, he rejected the final communiqué of the Group of Seven industrial nations just hours after a White House official sent a statement to reporters saying he was in favor of it, following a public spat with Canada’s Mr. Trudeau over trade.

“I should have a nickel for every foreign minister who has asked me to help decipher and interpret this administration,” said Richard Haass, who held foreign-policy positions in the past two Republican administrations. “They just don’t know how to read it. It’s been a nightmare for ambassadors and foreign ministers.”

Foreign officials have questioned whether conversations they have held with the president’s cabinet were “representative of reality,” a former U.S. official who served under the Trump administration said.

“They used to tell me, ‘We don’t know what to believe, what is on TV or tweeted,’ ’’ or what the president’s top advisers say, the official said.

In some cases, leaders think they might be able to get a better deal by going directly to Mr. Trump, according to several administration and foreign officials.

After the summit with Mr. Kim last June, Mr. Trump announced at a news conference he was canceling military maneuvers with South Korea, leaving lawmakers confused about what he had promised.

President Ronald Reagan and Soviet leader Mikhail Gorbachev in Moscow in 1988. Photo: Bettmann Archive/Getty Images

Past presidents have found one-on-one meetings to be helpful for building relationships, even with countries that have had an adversarial relationship with the U.S. The late President Ronald Reagan met then-Soviet Union leader Mikhail Gorbachev four times between 1985 and 1988.

“I believed that if we were ever going to break down the barriers of mistrust that divided our countries, we had to begin by establishing a personal relationship between the leaders of the two most powerful nations on earth,” Reagan wrote in his autobiography.

Former President George W. Bush hosted Mr. Putin at his ranch in Crawford, Texas, in November 2001, and at his Maine family compound in 2007. At one point, the two rode on a speedboat with Mr. Bush’s father, former President George H.W. Bush.

Mr. Putin visiting President George W. Bush at his Crawford, Texas, ranch in 2001. Photo: LUKE FRAZZA/Agence France-Presse/Getty Images

The elder Mr. Bush told local station WGME-TV the week of the meeting that the visit gave the two leaders a chance to “talk frankly without a lot of straphangers.”

More recently, on Angela Merkel’s last visit to the White House in April 2018, advisers stepped out of a meeting room, leaving Ms. Merkel and Mr. Trump to carry on the informal discussions alone. Advisers grew curious when they still hadn’t emerged after more than 20 minutes.

They later discovered Mr. Trump had taken Ms. Merkel for a room-by-room tour of the White House, according to one person briefed on their discussion.

—John Lyons and Dion Nissenbaum contributed to this article.

Write to Vivian Salama at vivian.salama@wsj.com and Peter Nicholas at peter.nicholas@wsj.com