The top military U.S. military leader warned Russian President Vladimir Putin that an invasion of Ukraine would be "horrific" CNN reported.

"Given the type of forces that are arrayed ... if that was unleashed on Ukraine, it would be significant, very significant, and it would result in a significant amount of casualties," Chairman of the Joint Chiefs of Staff Gen. Mark Milley said at a Pentagon press briefing. "You can imagine what that might look like in dense urban areas, along roads, and so on and so forth. It would be horrific. It would be terrible. And it's not necessary. And we think a diplomatic outcome is the way to go here."

A Pentagon spokesman said that with tens of thousands of Russian troops massed near the Ukrainian border, 8,500 U.S. troops have been placed on alert for possible deployment, including units with medical support, aviation support, logistical support and "combat formation."

However, Milley said the U.S. has no intention of putting "offensive forces" anywhere to attack Russia. He went on to say that tension between Russia and Ukraine "is entirely engineered by Russia and President Putin."

Russia denies any plans to invade Ukraine.

"If it is up to the Russian Federation, there will be no war," Russian Foreign Minister Sergey Lavrov to Russian journalists. "We don't want a war."

A U.S. official told CNN that the administration welcomes that message but expects it to be backed up with action.

"We need to see Russia pulling some of the troops that they have deployed away from the Ukrainian border and taking other de-escalatory steps," the official said, CNN reported.