The White House said Biden spoke with Poroshenko by phone on August 19 and "relayed that the United States had sent a message to Russia that the world is watching and underscored the need to de-escalate the situation," RFE/RL reports.
Biden "also urged Ukraine to show restraint," the White House said, while both leaders expressed concern over a recent surge in fighting with pro-Russian separatists in eastern Ukraine.
Read alsoRussia sends 55th 'humanitarian convoy' to DonbasBiden's talk with Poroshenko came as the Pentagon downplayed recent hype about a buildup of Russian forces in and around Ukraine's Crimean peninsula, which Russia annexed in 2014 and where it has been holding troop exercises.
"I think we are seeing movements associated with the upcoming exercise. We are not seeing this massive buildup of forces that has been suggested," Pentagon spokesman Captain Jeff Davis said.
"We don't see this unicorn a lot of people are chasing, this idea that there's some massive short-term build up or movement about to happen."
