By Tom Perry and Laila Bassam
BEIRUT (Reuters) - A U.S. cruise missile attack on a Syrian air base may persuade President Bashar al-Assad to be more cautious with some of his tactics, but will not deter him and his allies from pressing a full-throttle military campaign to crush rebels.
It was the first time Washington has directly targeted Assad's government in six years of civil war, and has pushed the administration of President Donald Trump into proclaiming that Washington still wants Assad removed from power.
But the single volley of Tomahawk missiles was of such limited scope that it will reinforce the view held by Damascus and its allies that the United States is no more eager than before to take the sort of strong action needed to defeat him.
"Assad now knows there is a red line with regard to the use of chemical weapons. But I think he also probably just sees it as a slap on the wrist," said David Lesch, professor of Middle East history at Trinity University and an author on Syria.
"Assad has to recalibrate but not fundamentally change his military approach that they've been engaging in since the Russian intervention," Lesch said. "I really believe they are not feeling too bad today, if this is the extent of what the U.S. is going to do."
Damascus denies carrying out the chemical attack that provoked the U.S. response. The attack in the town of Khan Sheikhoun in the rebel-held province of Idlib near the Turkish border killed at least 87 people, 31 of them children.
Assad has responded with characteristic defiance, vowing to accelerate efforts to wipe out rebels he calls terrorists.
A joint command center representing his Russian, Iranian and Lebanese Hezbollah allies said the U.S. attack would only cause them to redouble their support for the Syrian government.
Air strikes have continued unabated since the U.S. attack on Friday. Eighteen people were reportedly killed in one strike alone in Idlib on Saturday. Though damaged, the Shayrat air base near Homs is partly operational and flights have taken off.
The base was largely evacuated before the U.S. strikes, after Washington forewarned Moscow, which in turn alerted the Syrian government, according to a senior military source in the alliance fighting in support of Assad.
Describing the U.S. attack as a "limited strike" that was quickly over, another senior ally of Assad in the region said toppling him did not seem to be a priority for Trump. "There is still no clear American policy on Syria," he said.
Though the attack had shown Trump to be unpredictable, a third official in the pro-Assad alliance did not yet see a major shift in the U.S. approach.
"Is this a strategic shift by the Americans? Do they want to get into a big problem with the Russians? I don't think there is a strategic shift."
SHIFT FOR TRUMP
Washington says it acted because Syrian aircraft bombed Khan Sheikhoun with sarin, a banned nerve agent that Damascus pledged to give up in 2013 after Trump's predecessor Barack Obama threatened to bomb as punishment for another alleged gas attack.
Moscow and Damascus say the deaths were the result of a Syrian air strike on a depot where rebels were making chemical weapons that then leaked into the town - a claim rebels deny and Washington dismisses as beyond credibility.
The attack marked a departure from the approach of Obama, who ran a large-scale air campaign in Syria against fighters from Islamic State, also known as ISIS or ISIL, but avoided direct entanglement in the parallel civil war to unseat Assad.
The Obama administration provided limited support for anti-Assad rebels, but never directly struck government targets after Obama called off such strikes four years ago, at a time when Trump also said attacking Assad would be a mistake.
Ahead of his election victory, Trump had attacked Obama's approach in ways that appeared to suggest he would back off of calls to remove Assad. He questioned the wisdom of backing rebels, suggested that Washington should work more closely with Russia to fight Islamic State, and noted that while he didn't like him, "Assad is killing ISIS".
The first two months of Trump's presidency passed with little said about Assad's government, while extra U.S. troops arrived to help Kurdish and Arab militias in northern Syria fight against Islamic State.
A few days ahead of the chemical attack, two top U.S. officials made their clearest pronouncements yet on Syria, saying Washington was not now focused on making Assad leave power and the focus was on defeating Islamic State.
Some analysts believe the March 30 comments by Secretary of State Rex Tillerson and U.S. ambassador to the United Nations Nikki Haley emboldened Assad ahead of the Khan Sheikhoun attack.
"I think they were over confident. I think they felt that they could certainly get away with it - if in fact Assad did order this - because Idlib is controlled by al Qaeda affiliates, and the Russians are striking there, and the U.S. has also struck there," said Lesch.
"NO OPTION BUT VICTORY"
Since the attack, Trump has struck a tough tone, saying "something should happen" with Assad, but not yet saying what that should be. Haley has made an about face on her previous remarks, saying on Saturday that Assad's removal is a priority.
Tillerson seemed to take a more patient stance in regard to Assad, saying on Saturday that Washington's first priority is the defeat of Islamic State. He has also said there was no role for Assad in Syria's future.
The Syrian opposition, which long accused the Obama administration of inaction, wants the U.S. attack to be the start of a more aggressive policy toward Assad. Syrian rebel groups said on Friday the U.S. "responsibility" did not end with the missile attack.
"We are waiting for the American administration to reveal its complete vision for the Syrian file," said prominent opposition politician George Sabra.
Assad, whose forces have been in a much stronger position since receiving military backing from Russia in 2015, continues to press his advantage in a war that has killed more than 400,000 people and driven half of Syrians from their homes.
Military pressure and siege tactics have forced rebels out of numerous strongholds in recent months, including eastern Aleppo and areas near Damascus.
The opposition says Assad is forcibly displacing his opponents to remote parts of Syria in deals that offer rebels safe passage out, calling it a policy of demographic change.
One such agreement moved ahead on Saturday as planned. Syrian state TV said the Waer district of Homs city area would be declared "free of weapons" this month. The evacuation is taking place in phases, with Russian oversight on the ground.
Several hundred more fighters left Waer on Saturday, which has been besieged for years, for northern Syria with their families. They are being taken by bus to rebel-held areas of northern Syria, accompanied by Russian forces.
A Russian general interviewed by Syrian state TV said the U.S. attack would not derail implementation of the deal.
Assad, in an interview before the U.S. attack, made clear that so-called local "reconciliation" agreements remain central to his strategy, along with military action. Citing recent rebel attacks in Damascus and Hama, he said there could be no "results" with opposition groups at U.N.-backed peace talks.
There is no "option but victory", he said.
(Writing by Tom Perry)