The two Presidents are due to hold detailed one-on-one talks on the sidelines of the summit.

Russian President Vladimir Putin and U.S. President Donald Trump have shaken hands during their first face-to-face encounter at the G20 Summit in Hamburg, the Kremlin said on July 7, 2017.

“They shook each other's hand and said that they would soon hold a separate meeting, would soon see each other,” Kremlin spokesman Dmitry Peskov told a conference call with reporters.

Mr. Putin and Mr. Trump are due to hold detailed one-on-one talks on the sidelines of the summit later on July 7, 2017.

When asked if Mr. Putin was looking forward to the talks and whether he had lots of questions for Mr. Trump, Mr. Peskov said he did.

The Russian leader had been fully briefed about Mr. Trump’s description on July 6, 2017 of Moscow’s behaviour as destabilising, Mr. Peskov added, and would take that and other remarks by U.S. officials into account.

The leaders are expected to delve into a series of vexing foreign policy issues, including the conflict in Syria and Russia’s provocations in Ukraine. But much of the focus both in Washington and Moscow will be on whether Mr. Trump broaches the issue of Russia’s meddling in the election.

Before the meeting on July 7, 2017, Mr. Trump tweeted that he was looking forward to the visit, saying there was “much to discuss.” During a news conference in Poland on Thursday, he again refused to accept the conclusion by multiple U.S. intelligence agencies that Russia interfered to try to help Mr. Trump win last November. Mr. Trump said it could have been Russia, but that other countries could have meddled, too. “Nobody really knows for sure,” Mr. Trump said.

U.S. lawmakers and federal investigators continue to look into Russia’s election interference, along with possible collusion between Trump campaign associates and Russian government officials. That puts Mr. Trump under intense scrutiny over how he handles the sit-down with Mr. Putin, a former Russian intelligence agent known to come to meetings like this well-prepared.

The White House has scheduled 35 minutes for the meeting, raising questions about how much ground the leaders can be expected to cover.

Mr. Trump, who likes to have neatly packaged achievements to pair with high-profile meetings, may seek some concessions from Russia to show he’s delivering progress and helping restore a once-productive relationship that he recently described as being at an “all-time low.” Mr. Putin would almost certainly want something in return.

The list of issues ranges from Syria to Iran to Ukraine, and now North Korea, following Pyongyang’s test this week of a missile capable of striking the U.S.

Russia wants the U.S. to return the two compounds in New York and Maryland that were seized by the Obama administration as punishment for election meddling. It also wants the U.S. to ease sanctions it imposed on Russia after Mr. Putin annexed the Crimean Peninsula, and over Russia’s support of separatist elements in Ukraine.

The U.S. wants a resumption of adoptions of Russian children by American parents, which Russia banned in 2012, along with an end to what it claims is intensifying harassment of U.S. diplomats and other officials stationed in Russia.

Lawmakers in both political parties say Mr. Trump must confront Mr. Putin over the election.

Several senior Democratic U.S. senators served notice on Thursday that Mr. Trump would be in “severe dereliction” of his presidential duty if he fails to confront Mr. Putin over the issue, telling Mr. Trump in a letter that he must make clear that Russia’s interference in U.S. democracy will not be tolerated.

“The upcoming elections cannot be a playground for President Putin,” said Minority Leader Chuck Schumer of New York; Dick Durbin of Illinois, the Senate’s No. 2 Democrat; and the top Democrats on the Intelligence, Armed Services, and Foreign Relations committees.

Rep. Adam Kinzinger, Republican-Illinois, a member of the House Foreign Affairs Committee, said this week that he will “raise holy hell” if Mr. Trump goes soft on Mr. Putin. “It is very important for us to make a statement that Russia does not meddle not just in our elections, here and the future, but in our allies,’” he said.

Every detail of the Trump-Putin meeting will be scrutinised, from their facial expressions to the color of their neckties to how they shake hands.

“I expect an Olympic-level of macho posturing between these two leaders,” said Derek Chollet, executive vice president and senior adviser for security and defense policy at the German Marshall Fund, a Washington think tank. “The big thing to watch will be what Putin asks for and what he offers in return and whether there’s a sense of receptivity on the president’s part.”

Before Mr. Putin, Mr. Trump will try to manage another rocky international relationship when he meets with Mexican President Enrique Pena Nieto.

Mr. Pena Nieto had been scheduled to visit the White House shortly after Mr. Trump took office, but he scrapped the trip at the last minute due to disagreement with Mr. Trump over the U.S. President’s insistence that Mexico pay for the wall he has vowed to build along the U.S.—Mexico border to deter illegal immigration. Mr. Pena Nieto insists Mexico will not pay.

Mr. Trump has vowed to tighten border security and crack down on undocumented workers and drug cartels, but he has been less firm on getting Mexico to pay for the wall.

The Putin meeting is the highlight of a hectic, four-day European visit for Mr. Trump, who addressed thousands of Poles in an outdoor speech in Warsaw, Poland, on Thursday. He met in Germany with Chancellor Angela Merkel, the summit host, and had dinner with two Asian allies Japanese Prime Minister Shinzo Abe and South Korean President Moon Jae-in to discuss North Korea’s aggression.

The Group of 20 gathering of the world’s leading rich and developing nations is the first since Mr. Trump announced that the U.S. would withdraw from the Paris climate agreement, deeply disappointing Ms. Merkel and other U.S. allies who had hoped to maintain momentum in battling climate change. Even as Mr. Trump has said in vague terms he would like to renegotiate the Paris accord, European leaders have vowed to press forward.