Donald Trump speaks with European leaders ahead of G20 summit this week

Russia and United States are still discussing the timing of the encounter between Trump and Putin

US President Donald Trump spoke with German and Italian leaders on Monday, a White House official said, ahead of a summit of the Group of 20 leading economies this week that could expose his sharp differences with world powers on trade and other issues.

Trump is preparing for the two-day G20 summit that starts in Hamburg on Friday, just over a month after a G7 summit in Sicily showed deep divisions between Trump and other Western leaders on climate change, trade and migration.

A fractious first NATO summit with Trump also left European allies wondering where the military alliance goes next.

Trump will hold separate meetings with various leaders in Hamburg, including host German Chancellor Angela Merkel, and a potentially difficult first meeting with Russian President Vladimir Putin.

Trump announced on Twitter that he would be talking on the phone with leaders from Germany, Italy and France on Monday. The White House later said in a statement that the calls would be with Merkel and Italian Prime Minister Paolo Gentiloni, but did not mention French President Emmanuel Macron.

Trump and Macron spoke last week when the US president accepted an invitation to attend Bastille Day ceremonies in Paris on July 14.

Representatives for the White House did not immediately reply to a request for comment. Representatives for Macron's office could not immediately be reached for comment.

In remarks last week, Merkel raised the prospect of an open clash with Trump at the Hamburg summit, although some Trump administration officials have played down the discord.

Heather Conley, a State Department official during the Republican George W. Bush administration, said the United States' European allies are still shocked after the G7 and NATO summits with Trump cast doubt over Washington's relation with its allies.

"There's concern he could improve it - but could also do further damage," said Conley, who is now with the CSIS think tank in Washington.

Russia and the United States are still discussing the timing of the encounter between Trump and Putin, a Kremlin aide said on Monday.

Since Trump was elected US president, Russian has been keenly anticipating his first meeting with Putin, hoping it would trigger a reset in US-Russia relations that plunged to post-Cold War lows under Trump's predecessor, Barack Obama.

But with Trump embroiled in controversy at home over his associates' potential links to Moscow, the session with Putin has become a minefield. Too warm a meeting would allow Trump's domestic opponents to accuse him of being a Kremlin stooge.

High-level US diplomat Thomas Shannon met on Monday with Russia's ambassador to Washington, Sergei Kislyak, to prepare for the Trump-Putin meeting, the State Department said.

They also discussed scheduling a new meeting between Shannon and Russian Deputy Foreign Minister Sergei Ryabkov, which Moscow cancelled in June in anger at expanded US sanctions on Russia.