KYIV—Both parties to an energy ceasefire announced Sunday by the President of the United States said they strongly support the agreement and would very much like to know what is in it.

"Effective immediately, there is an ENERGY CEASEFIRE in the War between Russia and Ukraine," the President posted Sunday. "Both parties have agreed."

Ukrainian officials confirmed they had agreed, in the sense that they found out. "We just read it as you did," one Ukrainian official told CNN. President Volodymyr Zelenskyy said the announcement "was news for me," and added that Ukraine is "awaiting details from the American side."

The American side was reportedly awaiting the same details. "We're monitoring the President's account closely, like everyone," said Doug Pratley, a senior fellow at the Center for Agreements Already In Effect. "The terms should be on there any minute. That's where we get all our treaties now."

Diplomatic experts called the arrangement the first ceasefire in history in which the negotiating table was a phone and every party sat on the same side of it. Under the new process, two nations at war are told they have agreed to something, and they then have 24 hours to find out what it was.

"In the old model you'd spend months in Geneva," Pratley said. "This is much faster. You just refresh."

At press time, all parties had turned on notifications.