WASHINGTON (AP) — The White House says it is increasingly concerned that the beleaguered regime in Syria might be considering use of chemical weapons against its own people and warned that doing so would “cross a red line.”

White House press secretary Jay Carney said Monday that U.S. officials are closely monitoring Syria’s proliferation of sensitive materials and facilities, as opposition to the Syrian government grows.

Carney said President Barack Obama believes that any use of chemical weapons by the Syrian government would “cross a red line” for the U.S.

A senior U.S. defense official said Monday that U.S. and allied intelligence have detected Syrian movement of chemical weapons components in recent days.

Carney said “the world is watching” Syrian President Bashar Assad and said he’ll be held accountable for his actions.