"There's always a transformation that takes place from a person who wins the presidency, and then once he assumes office he necessarily sees the world from a different perspective", said Edward Djerejian, a former USA ambassador to Syria and Israel who now directs Rice University's Baker Institute for Public Policy.
Russian Federation has consistently denied that Syrian forces used chemical weapons, claiming the incident at Khan Sheikhoun was caused by a hit on a rebel chemical weapons plant, a claim dismissed by the West.
Assad accused the US of trying to boost the morale of "terror groups" in Syria.
It comes as North Korea said the U.S. missile strikes on Syria were an "intolerable act of aggression" that "proves a million times over" that strengthening its nuclear program was the right choice, state media reported Saturday.
Asked by a reporter whether he intends to take any action with regard to Syria, Trump says, "You will see". But a spokesman for Russian President Vladimir Putin warned the strikes dealt "a significant blow" to relations between Moscow and Washington. He says that once the militant group is eliminated, the USA can turn to stabilizing Syria.
Russian Federation maintains a close political and military alliance with the Assad government.
The strikes marked the first time Washington has directly targeted Syrian government forces since the war began in 2011. Indeed, greater US determination to adopt a multipronged, multilateral effort to weaken Assad or crush Syrian opposition might make defeating the Islamic State harder.
But the Foreign Office confirmed on Saturday that the foreign secretary would no longer make the trip because "developments in Syria have changed the situation fundamentally".
Xi agreed to increase China's cooperation with the USA to pressure North Korea to end their nuclear program, Tillerson said.
Russia's deputy ambassador to the UN, Vladimir Safronkov, described the U.S. strikes as "illegitimate".
"Swaggering as a superpower, the U.S. has been picking only on countries without nuclear weapons and the Trump administration is no exception", the foreign ministry spokesman said, according to KCNA.
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But the government hasn't backed down. "We're going to go on struggling", she said Wednesday, "because the one who tires, loses". Tens of thousands of opponents of President Nicolas Maduro flooded the streets of vehicle .
"United States intelligence indicates that Syrian military forces operating from this airfield were responsible for the chemical weapons attack on Syrian civilians in southern Idlib Province, Syria, that occurred on April 4".
Iran, which has provided crucial military and political support to Assad, meanwhile called for a fact-finding mission to determine what caused the chemical attack in Khan Sheikhoun.
"We all know that Assad would not poison his own people", he said in an online video.
The surprise barrage of 59 cruise missiles in the early hours of Friday, UK time, was the first time the U.S. has struck directly against the Syrian government. In the absence of any effective military alternative to Assad, it would also require a much greater role for the USA military in Syria, and eventual responsibility for what happens after Assad's removal.
In Florida with the president, Treasury Secretary Steven Mnuchin said additional economic sanctions on Syria were being prepared.
Allies in the Middle East and Europe who panned Trump's efforts to ban Syrian refugees from the United States cheered his decision to strike against Syrian President Bashar Assad's military after viewing images of young children killed in the chemical attacks.
The US stood firm behind its retaliatory deployment of 59 Tomahawks on Shayrat Airbase, from where Syrian government forces launched a devastating chemical attack last Tuesday that horrified the world and killed at least 87. After months of allegations of ties between his election campaign and the Kremlin - the subject of current congressional and FBI investigations - Trump has found himself clashing with Putin.
The action is likely to be interpreted as a signal to Russian Federation, as well as countries such as North Korea, China and Iran - where Trump has faced foreign policy tests early in his presidency - that he is willing to use force.
The US, they said, had not acted in self-defence and had not sought the agreement of the Security Council, the United Nations body tasked with maintaining global peace and stability. The drone returned late in the day as citizens were going to a nearby hospital for treatment. Some accuse Moscow of applying a "scorched-earth policy" that targets hospitals, schools and residential areas more than frontlines to break the resolve of the anti-Assad insurgency. Earlier, U.S. military officials had said they were looking into whether Russian Federation participated, possibly by using a drone to help eliminate evidence afterward. The attack occurred in the Shuaib al-Zeker area, near where USA -backed Syrian fighters have been battling IS under the cover of coalition airstrikes. As his advisers continue working on details for a military strike, Trump remains uncharacteristically disciplined about any plans.
Trump, who is spending the weekend at his Florida resort, tweeted a brief explanation Saturday of why the military didn't strike the runways in its bombardment of the Syrian air field, writing, "they are easy and low-priced to quickly fix (fill in and top)!"