Secretary of State John Kerry called for a war crimes investigation into Russia and Syria for what he called a deliberate campaign to terrorize civilians in the Syrian civil war, further evidence that already-strained ties between the Cold War foes deteriorate further.

Kerry said Russia and the regime of President Bashar Assad “owe the world more than an explanation” for attacks made to “terrorize civilians and to kill anybody and everybody who is in the way of their military objectives.” He cited a recent attack on a hospital, one of several that has taken place since a cease-fire deal reached Sept. 9 collapsed, killing 20.

“These are acts that beg for an appropriate investigation of war crimes and those who commit these would and should be held accountable for these actions,” Kerry said in Washington on Friday alongside French Foreign Minister Jean-Marc Ayrault. “They’re beyond the accidental now, way beyond, years beyond the accidental.”

Russia immediately rejected the allegations, with the head of the foreign affairs committee in the upper house of parliament saying no evidence of wrong-doing has been shown.

“It’s another move in a very aggressive information war that the Americans are conducting against Russia,” lawmaker Konstantin Kosachyov said in a phone interview. “Before you make such serious allegations, some evidence must be presented at least. There is no such evidence.”

The developments accelerated an erosion in ties between Moscow and Washington that had been under strain since Russia’s annexation of Crimea in 2014 and seem poised to outlast the end of President Barack Obama’s term in January. A U.N. Security Council meeting Friday failed to reach agreement on a way forward in the Syria crisis.

Russia this week suspended a 16-year-old treaty meant to reduce the risk of nuclear proliferation and Deputy Defense Minister Nikolai Pankov said the military is revisiting a 2001 decision to close bases in Cuba and Vietnam, nations Washington has been reaching out to.

In an interview with a Danish broadcaster aired on Thursday, Assad also denied authorizing attacks on hospitals and said mistakes are committed in any war. “Giving orders to destroy hospitals or schools or kill civilians, this is against our interests,” Assad said.