(Reuters)

The United States said Tuesday that it has observed Syrian chemical warfare personnel visiting known production facilities, suggesting that President Bashar al-Assad’s government is preparing fresh strikes on the rebel-held north of the country.  

The White House warned late Monday that the Assad government would pay a “heavy price” for any such strikes, indicating publicly for the first time that it believes the Assad government is capable of launching new chemical attacks. 

Marine Maj. Adrian Rankine-Galloway, a Pentagon spokesman, said Tuesday that the activity was centered at least in part on one aircraft hangar at the central Shayrat air base. U.S. Tomahawk cruise missiles hit the base in an April 7 barrage of strikes that marked the first American military intervention against Assad’s forces in six years of war.  

Those strikes came after Assad’s military dropped sarin nerve agent on the northern town of Khan Sheikhoun, killing scores of civilians and leaving hospitals overflowing with hundreds more casualties. 

The Syrian government was supposed to have surrendered its chemical weapons to international inspectors after a 2013 sarin attack on the Damascus suburbs killed almost 1,000 people and drew the Obama administration to the brink of military action.

A member of Syria’s civil defense breathes through an oxygen mask after a suspected gas attack in the town of Khan Sheikhoun in April. (Ammar Abdullah/Reuters)

That a significant stockpile remained was “one of the worst-kept secrets in international diplomacy,” a European official said Tuesday, speaking on the condition of anonymity because of the sensitivity of the issue.

A U.S. intelligence analyst said Tuesday that U.S. officials have been monitoring the movements of senior personnel from “Branch 450” of the Syrian Scientific Studies and Research Center in recent weeks as those figures have visited known and suspected chemical weapons production facilities.

Syrian military defectors have described Branch 450 as a unit that prepares and transports chemical weapons to facilities that deploy them. They said the unit’s orders usually come from Assad’s inner circle. 

On Monday, satellite imagery showed a Syrian aircraft parked near a building associated with chemical weapons at Shayrat, said the analyst, who spoke on the condition of anonymity because he was not authorized to speak publicly. 

The United States struck the base in April after it was used to launch a chemical attack on Khan Sheikhoun. Bombs containing sarin hit the town early in the morning, leaving hundreds of people, the youngest still in diapers, writhing in pain and foaming at the mouth. Doctors and rescue workers reported at least 74 deaths and 600 people injured.

Ali Haidar, Syria’s minister for national reconciliation, denied that the government possesses chemical weapons and accused the White House of waging a “diplomatic battle” against Syria at the United Nations, the Associated Press reported.

In Washington on Tuesday, Nikki Haley, the U.S. ambassador to the United Nations, told a congressional panel that Monday’s unusual late-night White House statement was a direct warning to the Assad regime.

“It is very much letting them know we are not going to give you a pass on killing men, women and children,” Haley said.

Asked whether the focus on Assad’s targeting of civilians represents a widening of the U.S. mission in fighting terrorism in Syria and Iraq, Haley replied, “I don’t think we have to pick one or the other.”

“ISIS is always going to be our priority, but I think we should always be realistic about the dangers of Assad,” she said. ISIS is another name for the Islamic State.

Navy Capt. Jeff Davis, a Pentagon spokesman, said the White House statement was released Monday night as part of a “fast-moving train.” The government wanted to issue the warning quickly after concerns were raised that an aircraft at Shayrat may have been loaded with chemical weapons, he said. The airfield remains under observation.

European leaders rallied behind the White House as news of the Syrian preparations emerged. In London, British Defense Secretary Michael Fallon said his government would support U.S. military action to prevent a chemical weapons attack, although it had not seen the intelligence upon which the American statements were based. 

French President Emmanuel Macron’s office said he and President Trump agreed in a phone call Tuesday that they would work together on a common response in the event of a new chemical attack.

In Moscow, Kremlin spokesman Dmitry Peskov told reporters that Russia has no information about an impending chemical attack and warned that any retaliation against the Assad government would be “unacceptable.”

The Russian military intervened to shore up Assad’s crumbling armed forces in 2015, and Moscow has never accepted the U.S.-led coalition’s conclusion that the Syrian government was responsible for the chemical attack on Khan Sheikhoun. 

Peskov said Tuesday that “it is impossible, unlawful and absolutely wrong from the point of view of achieving a final Syrian settlement to put the blame on al-Assad without holding an inquiry.”

Separately Tuesday, the U.S.-led coalition said it is investigating reports that airstrikes targeting Islamic State infrastructure in the eastern city of Mayadin also killed scores of civilians. The coalition said the missions Sunday and Monday were “meticulously planned and executed to reduce the risk of collateral damage and potential harm to non-combatants.”

But monitoring groups said the strikes also destroyed an underground prison holding at least 57 people arrested for breaking the Islamic State’s hard-line laws.

The Islamic State is believed to have moved most of its leadership to Mayadin in Syria’s Euphrates Valley southeast of Raqqa, the group’s besieged de facto capital, according to U.S. intelligence officials.

Omar Abu Layla, director of the Deir Ezzor 24 news network, said many of the civilian dead were buried in mass graves.

“All the people in that prison were civilians, arrested because they showed their opposition,” he said.

Lamothe and Nakashima reported from Washington. Anne Gearan in Washington, Thomas Gibbons-Neff in Garmisch, Germany, and David Filipov in Moscow contributed to this report.