Washington (CNN) US satellites, surveillance aircraft, drones and ships have stepped up operations to monitor the movement of suspected Iranian anti-air and ballistic missiles inside Syria due to rising concerns they could be used to strike Israel in the coming days, according to half a dozen US officials CNN has spoken with in the last 24 hours.

US officials confirm internal assessments of the situation have contributed to the level of concern but say they have no concrete sense when and if Iran might use its position inside Syria to strike Israel.

The US continues to recognize Israel's right to self-defense and its view that Iranian weapons inside Syria could threaten Israel directly. But they are concerned that any Israeli strike could result in a counterstrike by Iranian elements inside Syria.

Defense Secretary James Mattis acknowledged the rising tensions at a Senate Armed Services Committee hearing on Thursday. In answer to a question from Senator Jack Reed (D-RI) about whether there's a risk tensions could escalate to engulf not only Syria but the rest of the region Mattis replied, "I believe the short answer is yes senator. I can see how it might start, I'm not sure when or where, I think that it's very likely in Syria because Iran continues to do its proxy work there ... I could imagine this sparking something larger."

"We have seen them trying to bring advanced weaponry into Syria on its way to Lebanese Hezbollah in southern Lebanon. Israel is not going to wait until those missiles are in the air. Will it be cataclysmic? I hope not, I hope Iran pulls back," Mattis told the committee.

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