Washington (CNN) There are increasing concerns Iran is on the cusp of an attack against Israel, several US military officials told CNN.

Intelligence is not clear on when an attack could come and what form it would take, they said, with one official noting that "if there is an attack it might not be immediately clear it's Iran."

The US is watching very closely to see if Iranian-backed actions could come from inside Syria or Lebanon, or even from inside Iran itself, though that would be considered a major military escalation.

The US-led coalition fighting ISIS inside Syria said Tuesday that they have seen "no change" in the behavior of Iranian backed forces inside Syria in recent days.

"We've seen no change ... we closely monitor all threats to our forces, and as you know we retain our right to self-defense if we need to, but we see no change," said UK Army Major General Felix Gedney.

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