His warning came amid increasing violence in the pro-Moscow separatist east and accusations that Russia is increasing its forces, replenishing munitions and building up military hardware in the region.
Pentagon spokesman Captain Jeff Davis said the United States was concerned by the increasingly strident rhetoric from both Ukraine and Russia.
"We are concerned about Russia's continued occupation of Crimea, and we are concerned about the heightened level of violence in eastern Ukraine," he said.
"What we don't see (is) this unicorn a lot of people are chasing, this idea that there's some massive short-term build up or movement about to happen."
Russia holds military exercises in the region each year, with the next one due next month.
"I think we are seeing movements associated with the upcoming exercise, we are not seeing this massive buildup of forces that has been suggested," Davis said, noting that the United States continues to monitor the situation closely.
Kiev and its Western allies accuse Moscow of trying to escalate a 28-month conflict in Ukraine's rust belt that has claimed more than 9,500 lives and began just weeks after Russia annexed the Black Sea peninsula of Crimea in March 2014.
Poroshenko attended a summit of NATO leaders in Warsaw last month in which the alliance agreed to bolster its eastern flank in order to calm fears of Russia in both Ukraine and among other east European states.