The Pentagon is looking for any evidence that the Russian government knew about or was complicit in the attack in Idlib province that killed at least 80 people and injured dozens more, a senior US defense official said.
When CNN asked about US allegations that Russia may be complicit, Kremlin spokesman Dmitry Peskov replied via text message, "That is not true."
Russia earlier this week denied a chemical weapons attack took place in Syria. It said the deaths in Khan Sheikhoun were caused by a Syrian regime airstrike on a rebel-controlled chemical weapons factory on the ground.
Western leaders backed the US action, saying Syrian President Bashar al-Assad had brought it on himself.
- Pentagon probing possible Russia involvement in attack
- Syrian airbase suffers "extensive damage" in strike.
- Nikki Haley to UN: US 'prepared to do more' in Syria
- Trump acted after dozens killed in chemical attack on Tuesday.
- Western allies offer strong backing, saying the gas attack cannot go unpunished.
- Russian President Vladimir Putin said US strike violated international law.
The US military official said the Pentagon was examining specifically whether a Russian warplane had bombed a hospital in Khan Sheikhoun five hours after the initial chemical attack, with the aim of destroying evidence.
A US defense official says intelligence shows a Russian drone flew over the hospital in Idlib that was treating victims of the chemical attack, prior to the site being later bombed by an unknown aircraft.
The Russians operate drones in the area routinely so the Pentagon cannot be certain the drone operator even knew what was happening, but the drone was a Russian asset. The US military has a variety of classified technical means to determine who is operating aircraft in the region.
Russia said the deaths in Khan Sheikhoun were caused by a Syrian regime airstrike on a rebel-controlled chemcial weapons factory on the ground, but experts have dismissed the claim as fanciful.
Images of the horrific aftermath of the attack prompted Trump to act. Early Friday morning, Tomahawk cruise missiles were launched from US warships in the eastern Mediterranean
towards the Shayrat airfield, which is also an important base for Russian forces supporting the Syrian regime. Fifty-nine of 60 missiles launched hit their intended targets, Pentagon spokesman Capt. Jeff Davis said, with one missile landing in the water.
Nine people, including four children, in two villages were killed in the strike, Syrian state news agency SANA reported. It was not clear whether the figure included the six deaths announced earlier by the Syrian military or was in addition to those.
Approximately 20 Syrian regime planes were destroyed in the US strike, according to two US senior military officials.
Putin denounced the US action
as "aggression against a sovereign state in violation of the norms of international law under a far-fetched pretext." He said the strikes "dealt a serious blow to Russian-US relations" and that their aim was to distract from the civilian deaths from coalition airstrikes in Iraq, a statement from his press office said.
Russian Foreign Minister Sergey Lavrov accused the United States of seeking a pretext for regime change. "I am particularly disappointed by the way this damages US-Russia relations," he said, but added that he didn't think it would "lead to an irreversible situation."
Syrian President Bashar al-Assad said the United States had carried out an "unjust and unabashed assault" against Syria which "shows nothing but short-sightedness, a narrowness of vision and a blindness to political and military realities."
He also said the attack had increased the regime's resolve to "crush" terrorists in Syria -- the term it uses for all opposition forces.
A statement from Syria's general military command said the strikes caused "extensive material damage" and undermined counterterror operations by the Syrian army.
The operation "makes the United States of America a partner of ISIS, Nusra Front and other terrorist organizations who -- since the first day of this unjust war on Syria -- have been attacking Syrian army positions and Syrian military bases."
Syrian TV aired footage of the aftermath of the strike that showed smoke still rising up from the base.
Maj. Issam al-Reis, spokesman for the opposition Free Syrian Army's Southern Front, welcomed the US action and called for "the destruction of all tools of murder that Bashar al Assad's regime uses."
The strike was the first direct military action taken by the US against the Assad regime since the start of the country's six-year civil war. It represents a substantial escalation of the US military campaign in the region.
Pentagon: 'Severe damage'
The Pentagon said the strike, which began at 8:40 p.m. ET Thursday (3:40 a.m. local time Friday), targeted aircraft, storage facilities and other logistical materials.
An initial battle damage assessment from the strikes was that 58 of the 59 missiles "severely degraded or destroyed" their intended targets, according to a US defense official.
The US military gave the Russian military one hour's notice of the strikes, according to a senior US official. US military operations in Syria have been temporarily "adjusted" to ensure the protection of US forces on the ground, the official said. On any given day there could be up to 1,250 US troops inside Syria fighting ISIS, according to US officials.
Support from US allies
UN Secretary General Antonio Guterres, who said he was following the situation in Syria "with grave concern," urged restraint and called for the Security Council to unite and exercise its "responsibility for international peace and security."
The council's 15 members failed Thursday night to reach agreement on a a new draft resolution against the Syrian government circulated by US, UK and French diplomats following Tuesday's chemical attack. Russia has repeatedly vetoed past UN Security Council resolutions on Syria.
There were repeated calls for a political solution at a Security Council meeting Friday.
UK Ambassador to the United Nations Matthew Rycroft said his country backs the airstrikes "because war crimes have consequences and the greatest war criminal of all, Bashar al-Assad, has now been put on notice."
He said Russia "has given Assad everything he could dream of" -- such as vetoes in the Security Council -- but Assad has ignored and defied Russian requests to "obey a ceasefire and not to gas his people.
"Russia sits here today humiliated, by its failure to bring to heel a puppet dictator, entirely propped up by Russia itself and Hezbollah and Iran."
Vladimir Safronkov, Russia's deputy ambassador to the UN, defended his country's position on the strike, as a "flagrant violation of peace." He also slammed Rycroft's criticisms of Russia's government as "not diplomatic" and "lies."
"We strongly condemn the illegitimate action by the US. The consequences of this for regional and international stability could be extremely serious."
US Ambassador to the UN Nikki Haley defended the strikes as proportionate. "The United States took a very measured step last night," she said. "We are prepared to do more. But we hope that will not be necessary. It is time for all civilized nations to stop the horrors that are taking place in Syria and demand a political solution."
Matthew Chance, CNN's senior international correspondent based in Moscow, said the latest strikes "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."
Russia was warned of the attack before it took place. 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" which, he said, "implies a degree of tacit Russian consent to the strikes."
In the wake of the strike, Russia said it was suspending use of a communications channel aimed at minimizing risks of in-flight incidents between the US-led coalition fighting ISIS in Syria and Russian aircraft.
But on Friday, a senior military official told CNN the Russian military has not suspended "the deconfliction channel." The official said the channel was used just after the US strike on the Shayrat airfield and again Friday. No information on the content of the communications was given.
US military spokesman Col. John Dorrian said the "deconfliction channel" had been effective and urged its continued use.
The Russian Defense Ministry also announced plans to bolster the effectiveness of its air defense system in Syria.