Russian President Vladimir Putin condemned American air strikes on Syria in response to an apparent chemical attack as an act of “aggression against a sovereign state” and suspended an agreement with the U.S. to avoid hostile incidents in the skies above its Syrian ally.

The sudden escalation between the two nuclear-armed powers catapulted tensions to the highest level since Donald Trump took office in January. The Kremlin said the U.S. action will cause “considerable damage” to ties between Moscow and Washington. The U.S. said it had worked to minimize the risk of causing Russian casualties in the attack, which killed at least six Syrian servicemen.

The Shayrat Airfield was hit early Friday morning by 59 Tomahawk cruise missiles fired from the USS Porter and USS Ross, two Navy destroyers in the Mediterranean Sea, in what the U.S. said was a limited strike against the airbase from which the suspected chemical attack was launched.

Trump’s military move, which he said was a necessary response to an “affront to humanity” after more than 70 people died in a poison gas attack in northwest Syria on Tuesday, puts the U.S. and Russia into a potentially dangerous stand-off as Moscow stands behind Assad after six years of civil war.

Russia suspended a cooperation pact with the U.S. aimed at avoiding incidents between the two countries’ planes in the crowded airspace over Syria by establishing direct hotlines between their militaries.

“This is very risky,” said Andrei Kortunov, head of the Russian International Affairs Council, a Moscow-based research group set up by the Kremlin. “This agreement helped to avoid direct military confrontation in this difficult situation. We don’t want to start World War III.”

Russian Foreign Minister Sergei Lavrov said he hoped that what he described as “provocations” wouldn’t lead to “irreversible results.” Drawing a parallel to the 2003 U.S. invasion of Iraq, he accused the U.S. and its allies of seeking to sabotage Russian efforts to secure a peace deal in Syria and achieve regime change by the use of force in televised comments from Tashkent.

Until recently, Syria had seemed the one area where Putin and Trump were certain to find common ground. Trump during his election campaign had pledged to cooperate with Russia in fighting terrorism and top officials have made it clear the U.S. no longer seeks regime change in Syria.

The American leader was forced into action after the Washington anti-Russian establishment boxed him into a corner and is increasingly resembling his defeated Democratic opponent, Hillary Clinton, said Oleg Morozov, a former top Kremlin official who’s now a member of the upper house of parliament’s international affairs committee.

Syria denied using poison gas and Russia’s defense ministry has blamed the rebels, saying a Syrian air strike hit a chemical stockpile controlled by the armed opposition. The Kremlin said Friday that the Syrian government doesn’t have any chemical weapons stored.

Secretary of State Rex Tillerson, who is expected to meet Putin on his first visit to Moscow next week, blasted Moscow’s support for Assad, saying “either Russia has been complicit or Russia has been incompetent in its ability to deliver” on a 2013 deal in which it guaranteed that Syria gave up its chemical weapons.

Russian state television reported from the airbase Friday, saying the attack had not damaged the runway but had destroyed nine planes.

Russian forces so far have not been placed at risk by the U.S. actions, said Frants Klintsevich, the deputy head of the defense and security committee in the upper house of parliament.

“But if we see a threat to our bases or our servicemen, we of course will put the airspace in order,” he said by phone. Russia has its most advanced S-400 air-defense systems in Syria to protect its bases, which include a naval facility and an airbase.

Russia sees the U.S. action as a “symbolic gesture by Trump to show that the U.S. is back in the game, and that the U.S. and not Russia is a real superpower,” said Ruslan Pukhov, head of the Center of Analysis of Strategies and Technologies in Moscow, which advises the Defense Ministry.

“He needed to react to the chemical-weapons situation. He also sends a message to Putin — you fired so many of your Caliber missiles and now your workers are sweating to replace them, while we can waste dozens of our missiles on a simple gesture,” Pukhov said by phone, referring to the cruise missiles Russia has fired at targets in Syria.

Some voices in Moscow are questioning whether rogue elements in the Syrian regime could have been responsible, which would put Russia in a very awkward position as Assad’s main backer.