(CNN) — Russian President Vladimir Putin has denounced the US strike against a Syrian government airbase as “aggression against a sovereign state in violation of the norms of international law.”

The Russian Defense Ministry said Friday that it plans to bolster and increase the effectiveness of the air defense system in Syria following the attack.

“In order to protect the most sensitive objects of the Syrian infrastructure, a system of measures to bolster and increase the effectiveness of the Syrian armed forces’ air defense systems will be implemented,” ministry spokesman Igor Konashenkov said in a statement carried on state media.

Russia earlier announced it would suspend an October 2015 agreement aimed at minimizing risks of in-flight incidents between Russian and US-led-coalition aircraft. As a result, the risk of a direct aircraft collision between the US and Russia had “significantly increased,” Kremlin spokesman Dmitry Peskov said.

In a regular call with reporters, Peskov added that it was “indisputable” that the US airstrike on Syria “was carried out for the benefit of ISIS and other terrorist organizations.”

US President Donald Trump ordered the strike against the airbase from which a chemical weapons attack was launched earlier this week, US officials said. (Watch President Trump’s statement here)

In an earlier statement, the Kremlin said Syria “has no chemical weapons,” pointing to reports by UN inspectors.

“Vladimir Putin believes that complete disregard for factual information about the use by terrorists of chemical weapons drastically aggravates the situation,” it said in a statement.

“This move by Washington has dealt a serious blow to Russian-US relations, which are already in a poor state.”

The Kremlin added that Putin sees the latest US military action as “an attempt to distract from the mounting civilian casualties in Iraq.”

Russia is Syrian President Bashar al-Assad’s most powerful ally and has provided the military might behind his continuing grip on the country, which plunged into civil war six years ago.

No deal

The strike dashed any hopes for a US-Russia anti-terror coalition in Syria, Russian Senator Konstantin Kosachev, who heads the Federation Council’s international affairs committee, said.

“And it started so well, what a shame,” he wrote Friday on Facebook.

“In one way or another, Russian cruise missiles continue to strike terrorists, and American ones strike government forces which, in fact, lead the war against terrorists.”

He continued: “I’m afraid that given such an approach, the desired Russian-American anti-terrorist coalition in Syria, so often talked about since Trump rose to power, will repose in the Lord before it has the chance to be born.”

Russian Foreign Ministry spokeswoman Maria Zakharova said Friday that Russia would suspend the 2015 agreement aimed at minimizing risks of in-flight incidents.

“US actions further destroy Russia-US relations,” she added. “If earlier it was explained using the need to fight against terrorism, now it’s an obvious act of aggression against the sovereign state of Syria.”

The airstrikes “are an immensely dangerous episode in the relationship between Russia and the United States, not least because they potentially bring into contact Russian forces who are on the ground in Syria and the US forces,” Matthew Chance, CNN’s senior international correspondent based in Moscow, said.

Russia was warned of the attack before it took place, in accordance with deconfliction policies between the US and Russia over military activities in Syria.

Nevertheless, Chance pointed out, Russia did not choose to use surface-to-air missiles systems in place in Syria that are “fully capable of intercepting cruise missiles if they so choose to.”

“(This) implies a degree of tacit Russian consent to the strikes,” Chance said.

Russian involvement

Ahead of the strike against the airbase, Russian officials warned the US against making any “snap judgments” regarding the chemical weapons attack, which struck a rebel-held town Tuesday in Syria’s Idlib province.

Russia has attempted to negotiate ceasefires in Syria, and in January began withdrawing some military forces from the conflict area.

Support from Russian airstrikes for Syrian regime forces have been crucial in helping them to gain the upper hand in the long-running conflict and pushing rebel fighters from the key city of Aleppo in December.

But Russia’s next moves are uncertain, and prior to the US action, Putin had made it clear he expected Russian involvement to wind down.

The Kremlin has worked closely with Turkish President Recep Tayyip Erdogan, an enemy of Assad, to negotiate ceasefires and bring about a potential diplomatic solution to the conflict.