WASHINGTON (AFP) - The Syrian regime may have had help conducting a chemical strike in Syria, US officials said Friday (April 7), but they stopped short of accusing Moscow of complicity.

"We suspect that they had help" in Tuesday's attack on a rebel-held town on Tuesday (April 4) widely blamed on the Damascus regime, a US senior official told reporters.

"At a minimum, the Russians failed to rein in the Syrian regime activity," the official said, speaking on condition of anonymity.

"We know the Russians have chemical expertise in country. We cannot talk openly about any complicity between the Russians and the Syrian regime in this case, but we are carefully assessing any information that would implicate the Russians knew or assisted with this capability."

US forces fired 59 Tomahawk missiles at the Shayrat airfield near Homs in western Syria early Friday (April 7) local time in what it said was a proportional response for the attack in Khan Sheikhun.

The Pentagon says the base was used by the plane that launched the attack, and that it was known historically to have been a chemical weapons facility.

A second senior military official described how Russian personnel were closely enmeshed in the day-to-day operations at Shayrat, although he could not say if the Russians knew for sure that the Syrian jet that unleashed the attack was carrying a chemical payload.

The Russians "would have some type of information of any flight operation going out of that airfield on a daily basis," the official said.

US officials also said a small Russian or Syrian drone had been spotted surveilling a hospital that was subsequently destroyed after the initial chemical attack.

"We don't know who struck that," the first official said. "But the fact that somebody would strike the hospital, potentially to hide the evidence of a chemical attack... is a question that we are very interested in."