A military confrontation between Israel and Iran in Syria is becoming increasing likely, US Defence Secretary James Mattis warned on Thursday as his Israeli counterpart was visiting Washington to discuss the threat.

Mr Mattis delivered his warning of yet another military conflict in the Middle East at a hearing in Congress. Asked if Israel and Iran were edging toward military confrontation, he said: “I can see how it might start, but I am not sure when or where.”

"I think that it's very likely in Syria because Iran continues to do its proxy work there through Hezbollah.”

He accused Iran of not only expanding and strengthening its presence in Syria but also “bringing advanced weapons for Hezbollah through Syria”.

Israel, he said “will not wait to see those missiles in the air and we hope Iran would pull back”.

The Israeli Defence Minister Avigdor Lieberman, who met US national security adviser John Bolton and Mr Mattis on Thursday, also warned of a confrontation with Iran.

"Any site in which we see an Iranian attempt to achieve a military foothold in Syria will be struck. We won't let that happen, regardless of the price," he told the Arabic news website Elaph.

Israel said it carried out air strikes against Iranian and Syrian targets in Syria in February after one its F16 fighter jets was downed near the Syrian border.

Investigators examine the wreckage of an Israel F-16 that crashed in the northern Israeli Kibbutz of Harduf on February 10, 2018 after coming under fire by Syrian air defences. Jack Guez / AFP

Charles Lister, a senior fellow at the Middle East Institute who has closely observed the Syrian conflict, said the Iranian presence in Syria was becoming a “hugely explosive issue”.

“The threat is unacceptable and Israel can’t continue to watch it grow,” Mr Lister told The National.

The number of Iranian personnel in Syria has not increased, but the scale of Iran’s military infrastructure, permanent air activity, airbases, and weapon assembly factories within Syria has, he said.

“What it means is they [Israelis] need to do something,” Mr Lister said, and so “we started to see more aggressive, more risky strikes”.

He warned that with Russia unable to restrain Iran, and shipments of advanced weaponry to Hezbollah, there was a likelihood that confrontation in Syria could spiral out of control and engulf Lebanon.

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The talk of military escalation on the Syrian front was paralleled with more tamped down rhetoric on the Iran deal by Mr Mattis, who said Donald Trump had not made a decision on withdrawing from the deal, and that national security meetings were being held at the White House to review options ahead of the president's May 12 deadline.

Mr Mattis praised the verification process in the deal. “I have read it now three times, all 156 pages, and I will say it is written almost with an assumption that Iran will try to cheat. So the verification, what is in there, is actually pretty robust as far as our intrusive ability to get in.”

Also on Thursday, the Senate confirmed former CIA director Mike Pompeo at the 70th Secretary of State by 57 votes to 42. Mr Pompeo succeeds Rex Tillerson, who was sacked last month.

After his confirmation the State Department announced that Mr Pompeo would leave immediately for Brussels, Riyadh, Jerusalem and Amman, on a trip that concludes on Monday.

Mr Pompeo has promised to fill key vacancies in the department. Bloomberg reported on Wednesday that Paula Dobriansky will be nominated to replace Tom Shannon as undersecretary of state for global affairs. She has served in various capacities in the George H W Bush and George W Bush administrations.