Joint Chiefs of Staff Chairman Gen. Mark Milley on Wednesday spoke with his Russian counterpart amid heightened tensions over Moscow’s military presence at its border with Ukraine.

Milley spoke by telephone with Chief of Russian General Staff Gen. Valery Gerasimov to discuss “regional security-related issues of concern,” according to a readout of the call.

“The phone call is a continuation of communication between both leaders to ensure risk reduction and operational de-confliction. In accordance with past practice, both have agreed to keep the specific details of their conversation private,” the U.S. military said in the statement.

Russia has amassed roughly 100,000 troops at its border with Ukraine, sparking warnings from Western world leaders that Russia may move to invade the former Soviet state.

Russia has so far denied it is planning an invasion, and insists its forces are there in response to military moves by NATO which it claims are threatening.

Last week, Russia released draft security agreements demanding that the U.S. and NATO give legally binding guarantees that it will stop expanding further eastward, specifically by denying membership to Ukraine and other former Soviet bloc nations. Moscow also wants Western officials to promise that certain weapons would not be deployed to countries neighboring Russia.

The United States, meanwhile, has promised a “very meaningful and massive response” by Washington and its allies should Russia commit “renewed acts of aggression,” likely referring to the Kremlin’s 2014 annexation of the Crimean Peninsula.

“We’ve seen plans that Russia has to commit renewed acts of aggression against Ukraine that it could implement on very short order,” Secretary of State Antony Blinken said Tuesday.

Pentagon press secretary John Kirby also on Tuesday said the U.S. continues “to see a significant force presence near and around the Ukraine border” which “continues to be concerning.”

“We still don’t know the full intent here. And there’s no indication that [Russian President Vladimir Putin] has made a decision one way or another,” Kirby told reporters.

In an attempt to use diplomatic channels to quell the unrest, U.S., Russian and NATO officials are planning talks in January.

Moscow will also start separate negotiations with NATO in January, per Reuters.