As the US announced the new measures investigators were set to begin their work in investigating the chemical attack in Douma that prompted US-led airstrikes over the weekend.
President Donald Trump on Sunday defended his use of the phrase "Mission Accomplished" to describe a US-led missile attack on Syria's chemical weapons programme, even as his aides stressed continuing US troop involvement and plans for new economic sanctions against Russia for enabling Bashar al Assad's regime.
Stepping up the pressure on Syria's regime leader, US Ambassador Nikki Haley indicated the sanctions to be announced on Monday would be aimed at sending a message to Russia, which she said has blocked six attempts by the UN Security Council to make it easier to investigate the use of chemical weapons.
"Everyone is going to feel it at this point," Haley said, warning of consequences for Assad's foreign allies.
"The international community will not allow chemical weapons to come back into our everyday life," she said. "The fact he was making this more normal and that Russia was covering this up, all that has got to stop."
Haley's comments came as investigators from the Organisation for the Prohibition of Chemical Weapons were set to begin investigating the suspected chemical attack that was alleged to have taken place a little over a week ago and which prompted the US-led airstrikes over the weekend.
TRT World's Reagan Des Vignes reports.
Trump tweeted on Sunday that the strike was "perfectly carried out" and that "the only way the Fake News Media could demean was by my use of the term "Mission Accomplished."" He added that he knew the media would "seize" on the phrase, but said it should be used often. "It is such a great Military term, it should be brought back," he wrote.
Trump tweeted "Mission Accomplished" on Saturday after US, French and British warplanes and ships launched more than 100 missiles nearly unopposed by Syrian air defenses. While he declared success, the Pentagon said the pummeling of three chemical-related facilities left enough others intact to enable the Assad government to use banned weapons against civilians if it chooses.
His choice of words recalled a similar claim associated with President George W Bush following the US-led invasion of Iraq. Bush addressed sailors aboard a navy ship in May 2003 alongside a "Mission Accomplished" banner, just weeks before it became apparent that Iraqis had organised an insurgency that would tie down US forces for years.
Later Sunday, Trump sent a letter to congressional leaders informing them in writing of his decision to order the strike. Under the War Powers Resolution, the president must keep Congress informed of such actions.
Haley made clear the United States wouldn't be pulling troops out of Syria right away, saying US involvement there "is not done."
Haley said the three US goals for accomplishing its mission were making sure chemical weapons were not used in a way that could harm US national interests, defeating the Islamic State group and having a good vantage point to watch what Iran was doing.
"We're not going to leave until we know we've accomplished those things," she said.
Haley said the joint military strike "put a heavy blow into their chemical weapons programme, setting them back years" and reiterated that if Assad used poison gas again, "the United States is locked and loaded."
French President Emmanuel Macron said on Sunday that France wanted to launch a diplomatic initiative over Syria that would include Western powers, Russia and Turkey. Speaking on French television BFM and online site Mediapart, Macron stressed that the French diplomacy was able to talk with Iran, Russia and Turkey on one side and to the United States on the other side.
He said, "Ten days ago, President Trump wanted to withdraw from Syria. We convinced him to remain."
Asked about Macron's comments, White House spokeswoman Sarah Sanders stressed that Trump's plans for the region had not changed. In a statement, she said: "The US mission has not changed – the President has been clear that he wants US forces to come home as quickly as possible."
The night-time assault on Syria was carefully limited to minimise civilian casualties and avoid direct conflict with Russia, but confusion arose over the extent to which Washington warned Moscow in advance. The Pentagon said it gave no explicit warning. The US ambassador in Moscow, John Huntsman, said in a video, "Before we took action, the United States communicated with" Russia to "reduce the danger of any Russian or civilian casualties."
Russia has military forces, including air defences, in several areas of Syria to support Assad in his long war against anti-government rebels.
Russia and Iran called the use of force by the United States and its French and British allies a "military crime" and "act of aggression." The UN Security Council rejected a Russian resolution calling for condemnation of the "aggression" by the three Western allies.
Assad denies he has used chemical weapons, and the Trump administration has yet to present hard evidence of what it says precipitated the allied missiles attack: a chlorine gas attack on civilians in Douma on April 7. The US says it suspects that sarin gas also was used.
"Good souls will not be humiliated," Assad tweeted while hundreds of Syrians gathered in Damascus, the capital, where they flashed victory signs and waved flags in scenes of defiance after the early morning barrage.
The strikes "successfully hit every target," said Dana W White, the chief Pentagon spokeswoman. The military said there were three targets: the Barzah chemical weapons research and development site in the Damascus area, a chemical weapons storage facility near Homs and a chemical weapons "bunker" a few miles from the second target.
Although officials said the singular target was Assad's chemical weapons capability, his air force, including helicopters he allegedly has used to drop chemical weapons on civilians, were spared. In a US military action a year ago in response to a sarin gas attack, missiles took out nearly 20 percent of the Syrian air force, the Pentagon said.
The US-led operation won broad Western support. The NATO alliance gave its full backing; NATO Secretary-General Jens Stoltenberg said the attack was about ensuring that chemical weapons cannot be used with impunity.
In his televised address from the White House on Friday, Trump said the US was prepared to keep up the economic, diplomatic and military pressure on Assad until he ended a pattern of killing his own people with internationally banned chemical weapons.
That did not mean military strikes would continue. In fact, General Joseph Dunford, chairman of the Joint Chiefs of Staff, said no additional attacks were planned.
Asked about Trump's "Mission Accomplished" assertion, White said it pointed to the successful targeting of the three Syrian chemical weapons sites. What happens next, she said, is up to Assad and to his Russian and Iranian allies.
Haley appeared on Fox News Sunday and CBS' Face the Nation.