LONDON — The unexpected White House warning about a potential chemical attack raises the specter of more American military strikes on Syria.

No details were given Monday about the purported preparations or how they had been detected except that they were “similar” to those that preceded an April 4 attack on a rebel-held area of Idlib that killed at least 100 people, including 25 children.

If Syrian President Bashar al-Assad “conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price,” the White House warned.

So what is the evidence that Syria has use of deadly agents, how has the U.S. responded to such incidents in the past, and how have Syrian allies reacted to the White House's recent announcement?

How did Russia respond to the White House statement?

Officials in Russia — a key ally of the Assad government whose support has been crucial in helping the regime stay in power — said Tuesday that the U.S. was trying to find excuses for a military intervention.

"They have to somehow justify this military aggression against a sovereign state, so they are clearing the ground," said Andrey Krasov, deputy head of the defense committee in Russia's lower house of parliament, according to state-run RIA Novosti.

Frants Klintsevich, deputy head of the defense committee of Russia's upper house of parliament, told the same agency: "A new cynical, unprecedented provocation is underway."

He predicted the next move would be a U.S.-led strike on Assad's forces in order to change the course of the war. “The U.S. is readying a new attack on Syrian forces, that's obvious," he said.

Abdul-Hamid Alyousef holds his twin babies who were killed in Idlib on April 4. Courtesy off Alaa al-Youssef

Kremlin spokesman Dmitry Peskov told reporters: "I am not aware of any information [or] threat of the use of chemical weapons," adding that any such attack could be the work of anti-Assad rebel groups that Moscow describes as "terrorists."

"You know, there have been numerous cases of use of poisonous chemical substances by militant terrorists of ISIS and other criminal groups," Peskov said. "Perhaps, there is a potential danger of repeating such provocations. But once again, I'd like to repeat that I don't have exact information."

Does Syria have chemical weapons?

On Aug. 21, 2013, a sarin nerve gas attack killed hundreds of people in the Ghouta suburb of Damascus. The U.S. and other nations said Assad's government was responsible for what the U.N. later called a "war crime." Human Rights Watch said the government was the "likely" culprit and the rockets that carried the gas were likely fired from one or more of a string of military installations that ring the capital.

Later in 2013, Syria gave the Organization for the Prohibition of Chemical Weapons (OPCW) a declaration of its 1,300-ton chemical weapon stocks and facilities, which were then destroyed — a deal agreed between the administration of former President Barack Obama and Syria, brokered by Russia.

Nevertheless, in April this year, U.S. and U.N. officials said it’s possible Assad’s regime not only retained a significant stockpile of nerve agents — despite U.N. efforts to destroy them — but may also have regained its ability to manufacture more.

The OPCW reiterated in April that it was “not able to resolve all identified gaps, inconsistencies and discrepancies in Syria’s declaration and therefore could not fully verify that Syria had submitted a declaration that could be considered accurate” or that it was in compliance with the 2013 agreement.

While weapons production infrastructure was banned by the 2013 deal, there was nothing in the agreement to eliminate the scientific infrastructure — the cadre of scientists and small labs that could be used to re-establish a chemical weapons program.

A U.S. intelligence official told NBC News in April this year: "We have never taken the Assad regime at its word that it declared its entire chemical weapons stockpile. Assad has repeatedly shown that he is willing to use whatever chemical weapons he has retained or reconstituted to attack and terrorize his own people."

Despite the evidence that stocks or equipment still exist, Pentagon officials told NBC News in April that the U.S. has yet to find chemical weapons storage or production facilities that it can target. Syria was required to destroy all such facilities under the 2013 agreement, but creating small batches of sarin can be accomplished in much smaller facilities than were destroyed.

"It is clear that Assad still has a good deal of chemical weapons and the U.S. has presumably seen things that suggest bombs being loaded onto aircraft or similar," Hamish de Bretton-Gordon, a chemical weapons adviser and former commanding officer of the U.K. Chemical, Biological, Radiological and Nuclear (CBRN) Regiment, told NBC News.

He said: "Assad always seems to use them when in dire straits or try to get advantage."

After April's attack, the World Health Organization found that some of the injuries were “consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents.”

The OPCW said bio-medical samples collected from victims during their autopsy are “incontrovertible” evidence of exposure to sarin “or a sarin-like substance.”

Prior to that, OPCW fact-finding teams have been investigating other alleged uses of chemical weapons. A review of blood samples from two female victims of an attack in the area of Um Hosh on Sept. 16 last year indicated exposure to sulfur mustard.

Determining responsibility for attacks is left to a joint U.N.-OPCW investigative body known as the JIM. Last year, the JIM concluded that the Syrian government used chlorine gas in three attacks and ISIS extremists used mustard gas in one attack during 2014 and 2015.

Trace elements of sarin and the nerve agent VX were found at a facility that the Assad regime never declared in May 2015, according to Reuters.

How has the U.S. responded in the past?

Two days after the Idlib attack, the United States fired 59 Tomahawk cruise missiles at the Syrian airfield believed to have been used to carry out the strike.

Children receive treatment after a suspected toxic gas attack in Syria's northwestern Idlib province on April 4. Mohamed al-Bakour / AFP - Getty Images

President Donald Trump, who was once opposed to military action on Syria, said his attitude had changed and that the chemical attack crossed “many, many lines.”

“These heinous actions by the Bashar al-Assad regime are a consequence of the past administration's weakness and irresolution,” Trump said on April 4. "President Obama said in 2012 that he would establish a 'red line,' against the use of chemical weapons and then did nothing."

Before the August 2013 attack in Ghouta that killed close to 2,000 civilians and that the West blamed on the Syrian government, Obama had said the use of chemical weapons would serve as a "red line" for responding.

But Obama did not retaliate after Ghouta. Instead, Russia brokered the deal that saw Syria hand over tons chemical weapon stocks and facilities.

What does Syria say?

Assad's government has repeatedly denied using chemical weapons, a position echoed by Russia.

After the deadly gas poisoning in Idlib, Assad said accusations of a chemical weapons attack were “100 percent fabrication" used to justify American air strikes.

Russia said the deadly gas came instead from a leak in a weapons factory run by U.S.-backed rebels and that the episode has been used as an excuse for provocation.

"We are absolutely convinced that it was a provocation. Assad didn't use the weapons," President Vladimir Putin said in May. "It was done by people who wanted to blame him for that."

How are Russia and Iran involved?

Moscow and Tehran are important Assad allies, and his survival depends on their support.

Putin joined the military conflict decisively in 2015, launching air strikes against rebels that Russia says are backed by ISIS.

Russia has long had interests in Syria, particularly a key naval facility at the port of Tartous, which serves as Russia's sole Mediterranean base for its Black Sea fleet, and has forces at an air base in Latakia.

Iran’s elite Revolutionary Guards are fighting alongside government forces inside Syria, and last year helped to drive ISIS militants out of Palmyra for several months. Syria has also been used as an alleged route for weapons shipments to Hezbollah, the Lebanese Islamist militia and political party.

Mansur Mirovalev reported from Moscow. Robert Windrem from Washington, D.C.