The one-on-one meeting between Russian President Vladimir Putin and U.S. President Donald Trump is scheduled for Friday in Anchorage, Alaska.

Ukrainian President Volodymyr Zelenskyy and his European allies have stepped up efforts this week to prevent any deal between the U.S. and Russia from emerging at Friday's summit.

Trump: 25 percent chance of not being successful

Ahead of the meeting, Trump estimated that his upcoming discussion with Russian President Vladimir Putin has a 25 percent chance of not being successful.

In an interview with Fox News Radio, Trump described his meeting with Putin as "a chess game," adding that he believes Putin intends to make progress toward a deal on the Russia-Ukraine conflict.

The U.S. president downplayed talk of a ceasefire emerging from the summit and speculated about a possible second meeting involving more leaders.

"I think it's going to be a good meeting, but the more important meeting will be the second meeting that we're having. We're going to have a meeting with President Putin, President Zelenskyy, myself, and maybe we'll bring some of the European leaders along. Maybe not. I don't know that."

Trump said there would be a press conference after the talks, though he did not know whether it would be joint. He also noted that there would be "a give and take" on boundaries and land.

Putin: U.S. making 'quite energetic' efforts

In televised comments, Putin said the U.S. was "making, in my opinion, quite energetic and sincere efforts to stop the hostilities, stop the crisis, and reach agreements that are of interest to all parties involved in this conflict."

He added that these efforts aim "to create long-term conditions for peace between our countries, and in Europe, and in the world as a whole – if, by the next stages, we reach agreements in the area of control over strategic offensive weapons."

His comments signaled that Russia will raise nuclear arms control as part of a wide-ranging discussion on security when he meets with Trump.

A Kremlin aide said Putin and Trump would also discuss the "huge untapped potential" for Russia-U.S. economic ties.

Russia controls around a fifth of Ukraine, and Zelenskyy and European leaders worry that a deal could cement those gains, rewarding Putin for 11 years of efforts to seize Ukrainian land and emboldening him to expand further into Europe.

On Wednesday, Trump threatened "severe consequences" if Putin does not agree to peace in Ukraine and warned of economic sanctions should his meeting on Friday prove fruitless.

Zhang Hong, a researcher at the Institute of Russian, Eastern European, and Central Asian Studies of the Chinese Academy of Social Sciences, said potential sanctions would severely damage Russia's economy.

Zhang added that the threat signals the U.S. hopes to maintain high pressure on Russia before a peace agreement is reached, in order to facilitate a ceasefire between Russia and Ukraine.

Regarding the upcoming Alaska meeting, Zhang said it is clear that differences among Europe, Ukraine, the U.S. and Russia on the issue of peace remain serious.

Noting that Europe and Ukraine continue to insist on freezing the conflict, oppose any form of territorial exchange for peace, and reject the territorial exchange plan proposed by Russia, Zhang said achieving a major breakthrough in peace in the short term will be very difficult.

(With input from agencies)