The United States is weighing a multinational military response to the suspected Syrian poison gas attack as experts listed several key facilities that could be viable targets.
US President Donald Trump promised forceful action on Monday, saying a decision would be made soon following the suspected chemical weapons attack late on Saturday in the Syrian city of Douma that killed at least 60 people and injured more than 1000 others.
US officials who spoke to Reuters on condition of anonymity did not disclose any plans, but acknowledged military options were being developed.
The White House, Pentagon and State Department declined to comment on specific options or whether military action was likely.
Experts on Syria's war cited France and perhaps even Britain and Middle East allies as potential partners in any US military operation, which would aim to discourage future chemical weapons use in Syria's brutal civil war.
President Emmanuel Macron warned in February that France would strike Syria if it broke treaties prohibiting chemical weapons. France has more than a dozen warplanes in the Middle East and could look to strike from the sea.
British UN Ambassador Karen Pierce told reporters that Britain "would prefer to start with a proper investigation" but that all options were on the table and London was in close contact with its US and French allies.
Experts speculated that the retaliatory strikes, should they happen, would likely focus on facilities linked to past reports of Syrian chemical weapons attacks.
They cited potential strikes against bases including Dumayr air base, which is home to Syrian Mi-8 helicopters and has been linked in social media to the strike in Douma.
One US official, speaking on condition of anonymity, was unaware of any decision to go ahead with a strike, but said any plans for a possible attack could focus on targets associated with Syria's chemical weapons program while seeking to avoid anything that could spread poisonous fallout in civilian areas.
A more provocative strike could target Humaymim Airfield in northwest Syria, which was singled out by the White House in a March 4 statement that identified it as the starting point for bombing missions by Russian military aircraft in Damascus and Eastern Ghouta.
Speaking at a meeting with military leaders and national security advisers, Trump said he would make a decision by Monday night "or very shortly thereafter" on a response, adding that the United States had "a lot of options militarily" on Syria.
Two days after the attack, the White House was still only able to say that the attack fit the pattern of Syrian President Bashar al-Assad's chemical weapon use.
Initial US assessments have so far been unable to determine conclusively what materials were used in the attack and could not say with 100 per cent certainty that Assad's government forces were behind it.
The US envoy to the United Nations, Nikki Haley, said Washington "will respond" to the attack regardless of whether the UN Security Council acts or not.
The Syrian government and its ally Russia have denied involvement in the attack.
Australian Associated Press
Sign up for our newsletter to stay up to date.