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.
But the Israel Defense Forces (IDF) called up a limited number of reserves Tuesday evening following what the IDF termed the "identification of irregular activity of Iranian forces in Syria."
A spokeswoman for the IDF said it was a "specific call-up based on need."
Another IDF spokesman said "non-combat" reserves were being called up, pointing to the possibility that reserves serving in intelligence or medical roles were being mobilized.
The Israeli army has instructed authorities in the Golan Heights, in the north of the country, to open civilian shelters.
In a statement, the army said its troops are on high alert for an attack and defense systems have been deployed.
The statement warned that "any aggression against Israel will be met with a severe response" and that "the Israeli public should remain attentive to IDF instructions that will be given if necessary."
The US State Department also issued a security warning for American citizens on Tuesday, urging them to "consider carefully" travel to the Golan Heights "until the situation stabilizes."
US officials said they cannot yet draw a direction correlation between President Donald Trump's withdrawal from the Iranian nuclear agreement and a potential attack against Israel but assess that the growing rhetoric from both Iranian and Israeli leaders is causing an increase in concern from the US.
That rhetoric has escalated in recent weeks and has coincided with Trump hinting that he would leave the deal -- a decision he officially announced on Tuesday.
At the same time, Iran has covertly shipped missiles into Syria, according to the US.
Some Iranian-backed sites inside Syria have been bombed in recent days in attacks widely believed to have been carried out by Israel.
The US assessment now is that Iran could initially strike in retaliation against Israel for its airstrikes inside Syria.
Iran had already warned the US that withdrawing from the deal would be a "historic mistake."
"When it comes to weapons and defending our country, we will not negotiate with anybody," President Hassan Rouhani said.