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.
One US military official tells CNN that the Pentagon is also monitoring the situation constantly to determine if additional measures would be needed to protect US military assets and personnel in the region.
"The potential for escalation has grown," the defense official told CNN. The Pentagon is publicly trying to emphasize that any military moves by Iran "would take us where we don't want to go," the official said. "There is serious concern this could escalate."
Israeli Defense Minister Avigdor Liberman, speaking to the London-based, Saudi-owned Elaph newspaper, warned the Iranians on Thursday, saying, "If Iran strikes Tel Aviv, Israel will hit Tehran."
Liberman continued, "We will destroy every military site in Syria where we see an attempt by Iran to position itself militarily. If there is not silence in Tel Aviv and in Israel, there won't be any in Tehran."
Earlier this week CNN reported that US intelligence is monitoring a series of cargo flights from Iran into Syria, that the US suspects may be carrying weapons systems into Syria for potential use by Assad regime or Iranian forces.
While weapons shipments into Syria are not uncommon, these flights caught the attention of US intelligence because they occurred in the days after the April 13th US airstrikes on Assad regime targets.
When asked by reporters at the Pentagon if he agreed with the Israeli assessment that the weapons shipments from Iran to Syria are for the purpose of striking Israel, Mattis answered, "I can't think of any other purpose for them right now."
Israel reportedly struck several targets inside Syria earlier this month, including a T4 airbase in Homs province where Iranians had placed anti-aircraft missiles as well as unmanned aerial vehicles.
Israel has launched attacks on sites in Syria in the past, most notably in February when it struck twelve targets including three aerial defense batteries, as well as four targets which the Israel Defense Forces described as Iranian. That attack came after the shooting down of an Israeli F-16 fighter jet by Syrian forces, and the infiltration of Israeli airspace by what the IDF said was an Iranian drone.
A senior Iranian security official said Iran would punish Israel for the recent airstrikes on the T4 airbase in Homs, according to Iran's semi-official FARS news agency. Ali Shamkhani the Secretary of Iran's Supreme National Security Council, and former defense minister, said Israel has not yet realized that the era of "hit-and-run" has come to an end and it should pay the price for such "stupidity."
But Israel has also laid out its own red lines, including not allowing the transfer of high-powered weapons to Hezbollah; any breach of Israeli sovereignty; and working to prevent Iran from entrenching itself in Syria.
The base that was struck, known as T-4, has an Iranian presence, which Iran confirmed when the semi-official Fars News Agency said four Iranians had been killed in the strike. In addition, Israel says an Iranian drone that penetrated Israeli airspace in February was launched and controlled from this base.
Speaking to Israeli Army Radio, leader of the opposition Yesh Atid party, Yair Lapid, said: "T-4, which is the base we're talking about, is no longer just a Syrian base, it is a Syrian-Iranian base. And Israel has said -- and we are clear on this -- that Israel will not accept an Iranian military presence in Syria and will not accept (Iran's) creeping presence in Syria. This has a price."