A boy carries his belongings at a site hit by what activists said was a barrel bomb dropped by forces loyal to Assad in Aleppo’s al-Fardous district, Syria.

The Syrian Observatory for Human Rights said Syrian government warplanes dropped barrel bombs on rebel-held areas of Hama province on Tuesday, a day after the United States said their use could lead to further U.S. strikes in Syria.

A Syrian military source denied the Observatory report and said the army did not use barrel bombs – drums or cylinders packed with explosives and shrapnel that cause indiscriminate destruction on the ground.

The United States fired cruise missiles at a Syrian air base last week in response to a poison gas attack on a town in northwestern Syria, which Washington has blamed on government forces. The Syrian government denies responsibility.

The Observatory said “a number” of barrel bombs had been dropped on the towns of Taybat al-Imam and Soran north of Hama city in an area where rebel groups, spearheaded by militant factions, launched a major offensive last month.

Observatory director Rami Abdulrahman said relatively few barrel bombs were dropped. UN investigators have recorded regular use of such bombs by government forces in Syria.

The Syrian army said it had targeted “terrorist groups in the northern Hama countryside” near Soran, killing “a large number” of militants and destroying weapons including four tanks, artillery, and rocket-launching platforms. It did not say what type of weapons the army had used.

White House spokesman Sean Spicer on Monday mentioned barrel bombs alongside poison gas as weapons that were causing “babies and children” to suffer. “If you gas a baby, if you put a barrel bomb in to innocent people … you will see a response from this president,” he said.

The Syrian military source said: “We do not use these barrels and they do not exist in the Syrian Arab Army.”

Army operations were continuing across Syria, and “will not stop”, the sources said.

A military commander in the alliance fighting in support of Syrian President Bashar al-Assad said the army and allied forces launched an operation north of Aleppo city on Tuesday.

“The Syrian army and its allies began today a military operation, along several fronts, to gain control of the hills and some villages,” said the commander, a non-Syrian, speaking on condition of anonymity.

Assad has been backed in the six-year-old war by Russia, Iran and Shi’ite militias from countries including Iraq, Lebanon and Afghanistan.

“It’s a limited military operation that aims to isolate a focal point for the militants in the northern countryside” and reduce the shelling of Nubl and Zahra, two Shiite villages besieged by rebels in Idlib province, the commander said.

The Observatory said government forces bombarded several rebel-held areas north of Aleppo and clashed with the insurgents in the countryside.