Donald Trump will meet with his defence secretary and the chairman of his joint chiefs of staff this afternoon as the US considers possible strike targets in Syria.
James Mattis and general Joseph Dunford are heading to the White House, where discussions will centre around eight possible targets, including two Syrian airfields, a research centre and a chemical weapons facility, according to CNBC.
Trump has told reporters a decision will be made "fairly soon."
"We're looking very very seriously, very closely at that whole situation and we'll see what happens folks," he said.
The US President added: “Were having a number of meetings today, well see what happens. Now we have to make some … decisions, so theyll be made fairly soon.”
The US said late this afternoon it had blood and urine samples that had tested positive for an unnamed nerve agent. The officials said they were "confident" in the intelligence, according to reports.
French President Emmanuel Macron said France had proof the Syrian government carried out the attack, which aid groups have said killed dozens of people, and will decide whether to strike back when all the necessary information has been gathered.
“We have proof that last week … chemical weapons were used, at least with chlorine, and that they were used by the regime of (President) Bashar al-Assad,” Macron said.
In the UK, an emergency Cabinet meeting convened by Theresa May, in which she is thought to have discussed military action with her most senior ministers, lasted more than two hours.
Details are expected to be released this evening.
Earlier in the day, the Organisation for the Prohibition of Chemical Weapons (OPCW) backed the UK over Salisbury, confirming that the samples it has tested met "the identity of the toxic chemical" named as Novichok by the government.
The nerve agent – which the OPCW did not name in its statement – hospitalised former double agent Sergei Skripal, his daughter Yulia and a police officer. The latter two have since been discharged, while Sergei is said by doctors to be recovering and it is hoped he will soon also be discharged.
"The results of the analysis by the OPCW designated laboratories of environmental and biomedical samples collected by the OPCW team confirm the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury and severely injured three people," the independent body said.
Foreign secretary Boris Johnson said: "There can be no doubt what was used and there remains no alternative explanation about who was responsible – only Russia has the means, motive and record."
He added: "We will now work tirelessly with our partners to help stamp out the grotesque use of weapons of this kind and we have called a session of the OPCW executive council next Wednesday to discuss next steps. The Kremlin must give answers. We must, as a world community, stand up for the rules based order which keeps us all safe. The use of weapons of this kind can never be justified, and must be ended."
Last night Yulia Skripal issued a statement refusing contact with both the Russian Embassy and her cousin Viktoria, who lives in Russia and has been denied a visa by the Home Office.
This afternoon the Russian Embassy tweeted: "So far not a single post-4 March photo of Sergei and Yulia Skripal has emerged. This lack of transparency is here for a reason, and this reason is highly likely not protecting the Russian citizens."