Updated June 28, 2017 01:20:49

Photo: A baby with a homemade gas mask in Khan Sheikhoun in the aftermath of the deadly chemical attack. (Supplied: Yehya Alrejjo)

The White House says it has found "potential" evidence Syria is preparing for another chemical weapons attack.

Press secretary Sean Spicer issued an ominous statement on Monday that the US, "has identified potential preparations" for another chemical attack by the Syrian Government that it says, "would likely result in the mass murder of civilians, including innocent children".

He says the activities are similar to those made before an April chemical attack.

The Trump administration launched missile strikes in retaliation for that attack, which it blamed on Syrian President Bashar al-Assad.

Mr Spicer warned, "if Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price".

The Syrian Government dismissed the White House's latest allegations that it was preparing a chemical weapons attack, while Russia slammed what it described as "unacceptable" threats against "Syria's legitimate leaders".

Vladimir Putin's spokesman Dmitry Peskov specifically criticised the use of the phrase "another chemical weapons attack", arguing that an independent investigation into the April attack was never conducted despite Russia's calls for one.

The White House provided no immediate evidence to back up its latest claims.

Several State Department officials typically involved in coordinating such announcements said they were caught completely off guard by the warning, which did not appear to be discussed in advance with other national security agencies.

Typically, the State Department, the Pentagon and US intelligence agencies would all be consulted before the White House issued a declaration sure to ricochet across foreign capitals.

Mr Assad had denied responsibility for the April 4 attack on the town of Khan Sheikhoun in the rebel-held Idlib province that killed dozens of people, including children.

Days later, Mr Trump launched a retaliatory cruise missile strike on a Syrian government-controlled air base where US officials said the Syrian military had launched the chemical attack.

It was the first direct American assault on the Syrian Government and Mr Trump's most dramatic military order since becoming president months before.

Mr Trump said at the time that the Khan Sheikhoun attack crossed "many, many lines," and called on "all civilised nations" to join the US in seeking an end to the carnage in Syria.

Russia and Iran to also take blame, US says

The United States has taken a series of actions over the past three months demonstrating its willingness to carry out strikes, mostly in self-defence, against Syrian government forces and their backers, including Iran.

The missile attack came after years of heated debate and deliberation in Washington over intervention in Syria's bloody civil war.

The US ambassador to the United Nations Nikki Haley said on Twitter: "Any further attacks done to the people of Syria will be blamed on Assad, but also on Russia and Iran who support him killing his own people."

Since the April military strike, Washington has repeatedly struck Iranian-backed militia and even shot down a drone threatening US-led coalition forces.

The US military also shot down a Syrian jet earlier this month.

Mr Trump has also ordered stepped-up military operations against the Islamic State militant group and delegated more authority to his generals.

AP/Reuters

Topics: unrest-conflict-and-war, government-and-politics, united-states, syrian-arab-republic

First posted June 27, 2017 12:35:17