MOSCOW, Russia (AP) — Russia reacted to US military strikes on its ally Syria Friday by cutting a hotline intended to prevent mid-air incidents, a response that demonstrates Moscow’s readiness to defy Washington and could even put the two nuclear superpowers on a course toward military confrontation.

President Vladimir Putin signalled he was ready to risk a clash with the US and abandon hopes for mending ties with the US under President Donald Trump, rather than accept the humiliation of standing by while his ally is bombed.

Russia’s decision to suspend the hotline established after the launch of the Russian air campaign in Syria in September 2015 effectively means that Russian and US planes could fly dangerously close to each other during combat missions, raising the risk of inadvertent or deliberate clashes in the crowded skies over Syria.

By freezing the information channel between the two potent militaries, Russia is signalling to Washington that it will tolerate no further strikes on Syrian government facilities.

Syria has aging Soviet-built aircraft and air defence missile systems, while Russia has deployed dozens of its cutting edge warplanes and air defence batteries at its base in Syria’s coastal province of Latakia. It also has a strategically important naval outpost in the Syrian port of Tartus, which is protected by air defence assets.

Further upping the ante, the Russian Defence Ministry said it will now help strengthen Syrian air defences.

US officials accused Russia of failing to ensure Syrian President Bashar Assad’s commitment to a 2013 deal for the destruction of Assad’s chemical weapons arsenal. The US says that arsenal was tapped for a chemical attack that killed dozens of civilians in the Syrian town of Khan Sheikhoun in Idlib province.

Trump cited the chemical attack as justification for the missile strike on a Syrian air base. But the Kremlin insists Assad’s government wasn’t responsible for the attack, saying civilians in Khan Sheikhoun were exposed to toxic agents from a rebel arsenal that was hit by Syrian warplanes.

“President Putin believes that the US strikes on Syria represent an aggression against a sovereign state in violation of international law under a far-fetched pretext,” Kremlin spokesman Dmitry Peskov said in a statement. “Washington’s move deals a significant blow to Russia-US relations, which are already in deplorable shape.”

Until the attack on the Syrian air base, the US had avoided striking Assad’s forces for fear of provoking a clash with the Russian military.

The action comes ahead of US Secretary of State Rex Tillerson’s trip to Moscow next week.

The Kremlin initially had been encouraged by Trump’s goal of repairing ties with Moscow, which plunged to post-Cold War lows under President Barack Obama, but hopes for a thaw have withered amid the congressional investigation of possible links between Trump campaign officials and Russia. The US missile strike could make it all but impossible to improve relations.

“Some people here thought that it would be easy to deal with Trump,” Yelena Suponina, a Moscow-based Mideast expert, said in televised remarks. “No, it will be very difficult. He’s not only ready to make tough decisions, he is unpredictable.”

Mikhail Yemelyanov, a senior member of the lower house of parliament, warned that the US action raised the threat of a direct clash between Russia and the US.

“Consequences could be grave, up to military confrontation and exchange of blows, nothing can be excluded,” he said, according to the Interfax news agency.

Tillerson said Russia had “failed in its responsibility” to deliver on a 2013 deal it helped broker to destroy Syria’s chemical arsenal.

“So either Russia has been complicit, or Russia has been simply incompetent on its ability to deliver,” he said.

By ordering the strike, Trump threatened the military assets of Assad, who has enjoyed Russia’s support throughout the six-year conflict. Russia’s military has helped turn the war in Assad’s favour and Moscow has used its UN Security Council veto to protect Damascus from censure.

Russia also has important military facilities in Syria that could be put at risk if Assad is removed from power, a goal of Western powers that had recently been put on the back burner because of the focus on fighting Islamic State extremists in Syria and Iraq.

Peskov said the US gave Russia advance notice about the strike. He added that Moscow believes it makes no sense to maintain the hotline.

Asked if the decision to freeze the information exchange could raise the risk of mid-air incidents, Peskov said it was the US attack that increased such danger.

Peskov wouldn’t say if Russia could use its military assets to protect Syrian facilities from future US strikes.

Defence Ministry spokesman Major General Igor Konashenkov said Russia will quickly “strengthen the Syrian air defence system and increase its efficiency in order to protect Syria’s most sensitive infrastructure facilities.”