The United States on Wednesday condemned an alleged deadly strike by Syrian regime forces on a renowned rescue team, but refused to say why it had denied the group's leader a visa.

State Department spokesman Mark Toner expressed horror at what he said were "multiple aerial strikes" on a Syrian Civil Defense station Monday in the northern town of Al-Tarab near Aleppo.

"At least five members of the civil defense are believed to have been killed and many more innocent people were injured," he said.

Toner said the strikes reflect an "abhorrent pattern" in the tactics of Bashar al-Assad's regime, in which rescue and medical workers in rebel-held areas have been deliberately targeted.

The spokesman also paid tribute to the Civil Defense group, better known as the "White Helmets," for the work they have done during Syria's five-year-old civil war to save civilian victims of the fighting.

But he would not say why, earlier this month, US officials had barred the volunteer group's leader Raed Saleh from entering the United States when he arrived to collect a major humanitarian award. 

Toner said US law prevents him from discussing individual visa cases, but seemed to suggest that security considerations could have been in play.

"Broadly speaking ... on any visa case, we are constantly looking at new information," he said.

"And if we do have new information that we believe this individual ... would pose a security risk, we'll certainly act on that."

He would not say if Saleh is under suspicion, but added "any individual in any group suspected of ties or relations with extremist groups, or that we had believed to be a security threat to the United States, we would act accordingly."

Source :AFP