(Adds response to diplomatic effort)
NEWPORT, Wales, Sept 4 (Reuters) - The United States is preparing a new round of economic sanctions against Russia for its incursion into Ukraine, a senior White House official said on Thursday on the fringes of a NATO summit in Wales.
Deputy White House national security adviser Ben Rhodes gave no details of the sectors that would be targeted but told reporters that the new penalties were being finalized.
"The key point is that Russia must continue to face costs for its own escalation," he said. "If Russia escalates, we can escalate our pressure."
Earlier, Ukraine President Petro Poroshenko announced the main pro-Russian rebel leader said they would both order ceasefires on Friday, provided that an agreement was signed on a new peace plan.
Rhodes said the move had the strong support of U.S. President Barack Obama and other key European leaders from Britain, France, Germany and Italy.
"We all believe that this situation can be resolved diplomatically in a way that respects Ukraine's sovereignty and territorial integrity, that's acceptable to the government in Kiev," he said.
"That is the best outcome. The leaders made clear that they would continue to support President Poroshenko's very active diplomacy with President Putin."
The White House said earlier in a statement that the key European leaders and Obama had agreed that Russia should face "new costs" for its actions in eastern Ukraine. However, French President Francois Hollande said a new round of European Union sanctions, due to be adopted on Friday, would depend on events in the coming hours.
Poroshenko said he would order a ceasefire on Friday if a peace plan opening the way for a political settlement was signed in talks in Minsk. (Reporting by Steve Holland; editing by Michael Holden and Paul Taylor)