A warning from the administration of President Donald Trump that Syria's government will "pay a heavy price" for any future chemical weapons attacks came after it observed similar activities on a base that had allegedly been used previously for sarin gas attacks in April.

According to the Pentagon, the activity indicates that the regime of Syrian President Bashar al-Assad is actively pursuing preparations at Shayrat airfield to use such weapons in its ongoing civil war that has torn the country apart. The US contends that the Assad regime used chemical weapons two months ago in an attack launched from Shayrat that killed dozens of civilians, prompting swift retribution from American military forces that fired 60 cruise missiles at the base.

Pentagon spokesman Captain Jeff Davis said that the recent activity "involved specific aircraft in a specific hangar ... Both of which we know to be associated with chemical weapons use”.

Mr Davis said the activity occurred during “the past day or two”. He did not say how the United States collected its intelligence.

The White House has said that another round would likely result in the "mass murder of civilians, including innocent children" and that the Syrian government would pay a "heavy price" for another chemical weapons attack.

"As we have previously stated, the United States is in Syria to eliminate the Islamic State [Isis] of Iraq and Syria" a statement attributed to White House Press Secretary Sean Spicer read. "If, however, Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price".

The White House threat caught many in Mr Trump's own administration by surprise. Several State Department officials typically involved in coordinating such announcements said they were caught off guard, and it appeared the underlying intelligence information was known only to a small group of senior officials. Typically, the State Department, Pentagon and US intelligence agencies would all be consulted before a White House declaration sure to ricochet across foreign capitals.

"We want to clarify that all relevant agencies ... Were involved in the profess from the beginning," spokeswoman Sarah Huckabee Sanders said on Tuesday. She noted that the State Department, Pentagon, CIA, and Office of the Director of National Intelligence were all involved in the process. "Anonymous leaks to the contrary are false".

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

US ambassador to the United Nations Nikki Haley said that the warning wasn't just aimed at the Syrian military, but was also focused on deterring attacks from Russia and Iran. She said that she hopes the warning will make those two countries rethink their relationship with Mr Assad's regime.

Mr Assad is "barbaric", she said. I can't "see a healthy Syria with Assad in place", Ms Haley said.

US Senator Bob Corker, Republican chairman of the Senate Foreign Relations Committee, said the concerns about preparations for a chemical weapons attack were authentic. “The claims that they made, from my perspective, are valid claims,” he said.

Senator Tim Kaine, a Democratic member of the Foreign Affairs and Armed Services committees, who has been leading a bipartisan push for Congress to debate and vote on the use of military force in Syria, Afghanistan and elsewhere, said there was no legal justification for extracting a "heavy price" from Syria.

The 60 cruise missiles fired at Shayrat base in April exacerbated tensions between the United States and Russia, which has aligned itself with the Syrian regime in the civil war. Soon after the attacks, American officials, including the President himself, said that relations between the US and Russia were at a low point. Russian officials have also noted that the war in Syria is the biggest cause of tension between Washington and Moscow, and that US military attacks like the one in April raises the risk of confrontation between the two powers.

Following the latest warning to Syria on using chemical weapons, the Russian government dismissed the claims and called them "unacceptable".

"I am not aware of any information about a threat that chemical weapons can be used," Kremlin spokesman Dmitry Peskov told a conference of reporters. "Certainly, we consider such threats to the legitimate leadership of the Syrian Arab Republic unacceptable".

The Syrian government didn't immediately comment, although Syrian state-run television station al-Ikhbariya said the allegations were fabricated

Mr Assad has been pictured visiting the Russan air base Hmeymim in western Syria recently, marking his first visit to a base where Russian jets take off to support his war effort.

Those photos show Mr Assad in the cockpit of a Russian Sukhoi SU-35 warplane, and taking a look at an armoured vehicle. The base is central to Russia's strategy in the effort to prop up Assad, an effort that has been ongoing since 2015.