MOSCOW — Russia on Friday condemned a U.S. missile strike against Syrian government forces as an attack on its ally and said it was suspending an agreement to minimize the risk of in-flight incidents between U.S. and Russian aircraft operating over Syria.

Even as Russian officials expressed hope that the strike against Syrian President Bashad Assad’s forces would not lead to an irreversible breakdown in U.S. relations with Moscow, the Kremlin’s decision to suspend the 2015 memorandum of understanding on the air operations immediately raised tensions in the skies over Syria.

President Vladimir Putin’s spokesman said the risk of confrontation between aerial assets of the U.S.-led coalition and Russia has “significantly increased” after President Donald Trump ordered the launch of 59 Tomahawk cruise missiles at a Syrian air base in retaliation for a chemical attack that killed scores of civilians.

Under the now-suspended pact, the two countries had traded information about flights by a U.S.-led coalition targeting the Islamic State and Russian planes operating in Syria in support of the Assad government. Moscow was taking its action, the Russian Defense Ministry said, because it sees the U.S. strike “as a grave violation of the memorandum.”

Putin’s spokesman, Dmitry Peskov, further claimed that the Syrian government had no chemical weapons and dismissed the Trump administration’s explanation as an excuse to enter the conflict.

“President Putin considers the American strikes against Syria an aggression against a sovereign government in violations of the norms of international law, and under a far-fetched pretext,” Peskov told reporters. “This step by Washington is causing significant damage to Russian-American relations, which are already in a deplorable state.”

“Of course, Syria is our ally, considering that we are helping the Syrian armed forces at the Syrian leadership’s request,” Peskov said.

The strike creates the possibility of a direct confrontation with Russia, which has forces on the ground and advanced air-defense systems capable of shooting down U.S. aircraft and missiles.

The so-called “deconfliction” channel that Russia suspended was established in 2015 to prevent mishaps, including collisions, after Russia deployed aircraft to a base along Syria’s Mediterranean coastline and began carrying out strikes on behalf of the Syrian regime. It calls for a U.S. colonel at an air base in Qatar and a Russian colonel to man a phone hotline and inform each other of where their countries’ planes are flying.

The arrangement has been far from ideal, however, and U.S. military officials have called in recent months for an expansion of deconfliction talks as Russian and U.S. military aircraft fly in increasingly close quarters over Syrian cities such as Manbij.

Senior U.S. military officials have said they have resorted to flying advanced F-22 Raptor jets at the top of the “stack” formations used to carry out airstrikes in part because they can better keep track of incoming aircraft and direct other coalition planes to shift out of the way of incoming Russian aircraft.

Two U.S. military officials, speaking on condition of anonymity, said Friday morning that they were aware of Russia’s stated intention to suspend the channel, but it was not yet clear how losing it may affect air operations. The Pentagon was still assessing the situation, they said.

Before the missile strike, a Pentagon spokesman, Capt. Jeff Davis, said in a statement, “U.S. military planners took precautions to minimize risk to Russian or Syrian personnel located at the airfield.”

There have been no reports of Russian casualties in Friday’s strike, but Syrian officials claimed that civilians, including children, were killed in the attack.

In Moscow on Friday, a Russian Defense Ministry spokesman, Maj. Gen. Igor Konashenkov, said Russia would help strengthen Syrian air defenses to “protect the most sensitive Syrian infrastructure facilities.”

Konashenkov said the attack destroyed a warehouse, classrooms, a cafeteria, six Mig-23 fighter jets that were being repaired and a radar station. The runway and other aircraft were not affected, he said.

“Therefore, the military effect of the massive American missile strike on the Syrian air base was extremely small,” he said.

Maria Zakharova, a spokeswoman of Russia’s Foreign Ministry, also dismissed the U.S. assertion that the attacks were a response to this week’s chemical weapon attack in northern Syria, which left scores dead in a village in Idlib province – one of the last strongholds of anti-Assad factions.

“It is obvious that the strike by U.S. cruise missiles was prepared well in advance,” Zakharova said on Russian state television. “It is clear to any specialist that the decision to deliver the strikes was made in Washington before the Idlib events, which were simply used as a pretext for demonstrating force.”

Putin’s spokesman said the Russian president considered the attack an attempt to distract attention from the heavy civilian casualties caused by a U.S.-backed offensive to capture the northern Iraqi city of Mosul from the Islamic State group.

“This step is not taking us any closer to the final goal in the fight against international terrorism, but, on the contrary, it is creating a serious obstacle to the establishment of an international coalition to combat it and efficiently counter this global evil,” Peskov said.

Under a 2013 Russia-U.S. agreement, Syria agreed to dismantle its chemical weapons stockpile. A U.N. mission in 2014 confirmed that most of Assad’s “declared” chemical arsenal had been eliminated. But this week’s attack in Idlib raised questions about whether some arms were held back.

Secretary of State Rex Tillerson, who is due to arrive in Moscow next week, said the attack on Idlib meant that “clearly, Russia has failed in its responsibility to deliver on that commitment.”

“So either Russia has been complicit or Russia has been incompetent in its ability to deliver on that agreement,” Tillerson said at a briefing late Thursday.

Other Russian officials issued grave condemnations of the U.S. strike.

“This is fraught with the possibility of a clash between Russia and the U.S.A., and the consequences might could be the most severe, including military conflict,” Mikhail Yemelyanov, a member of the lower house of the Russian parliament, told the Interfax news agency. “Here you can’t rule out anything.”

“This will have negative consequences for normalization of the situation in Syria,” said Viktor Ozerov, the head of the defense and security committee of the upper house of the Russian parliament.

He said the attacks would send the wrong signal to Assad’s opposition and undermine the peace process.

Peskov had told the Associated Press on Thursday that Russia’s support for Assad was not unconditional, but he insisted that Russia wants to see a full investigation into the use of chemical weapons in Syria before the United Nations takes any action.

Putin carefully orchestrated a peace process that brought together Turkey and Iran – regional powers that have backed opposing sides in the civil war. At the same time, the chemical weapons attack suggested that Assad and his Iranian allies have no intention of being party to a power-sharing agreement with the opposition, indicating that Putin’s deal is all but dead.

U.S.-Russian relations are at their lowest point in decades, over Moscow’s annexation of Crimea and its proxy war in eastern Ukraine, as well as allegations that the Kremlin interfered in the U.S. presidential election last year.

Following the election of Trump, Russian leaders expressed measured optimism for an improvement in relations, but Peskov and others have said that so far there has been minimal dialogue. Trump did call Putin on Monday to express condolences after an explosion that Russian investigators have called a terrorist attack killed 14 and wounded dozens more in a St. Petersburg subway train.