U.S. Secretary of State Marco Rubio spoke by phone with Russian Foreign Minister Sergei Lavrov to prepare for the two presidents' summit on Friday (Aug. 15), although he reiterated that the meeting might not produce a breakthrough in the Ukrainian conflict.

"Both sides reaffirmed their commitment to ensuring the success of the event," U.S. State Department spokeswoman Tammy Bruce said in a statement. Bruce did not disclose the details of the call.

U.S. President Trump and Russian President Vladimir Putin are scheduled to meet in Alaska on Friday, part of the latest U.S. effort to broker a ceasefire in the Ukrainian conflict.

In a separate interview, Rubio reiterated Trump's characterization of the summit as an "exploratory meeting" that would help the administration decide whether to continue applying economic and military pressure to broker a peace agreement.

"We'll know early in the meeting whether this has a chance of success," Rubio said. "It's not going to be easy."

"The president agreed to this meeting at the request of President Putin," White House spokeswoman Carolyn Levitt said on Tuesday (Aug. 12). "The president's purpose in participating in this meeting is to better understand how we can end this conflict."