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 says was carried out by government forces. The 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 jihadist factions launched a major offensive last month.

Observatory director Rami Abdulrahman said the number of barrel bombs dropped by government warplanes was relatively small. U.N. investigators have recorded regular use of such bombs by government forces in Syria.

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 into 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.