Trump’s national security adviser John Bolton also says Washington seeks to change Iran’s behaviour, not its regime.

US President Donald Trump‘s national security adviser has warned that the United States would respond “very strongly” if forces loyal to Syrian President Bashar al-Assad use chemical weapons in an offensive to retake Idlib province.

Speaking at a press conference on Wednesday during a visit to Jerusalem, John Bolton said: “We now see plans for the Syrian regime to resume offensive military activities in Idlib province.

“We are obviously concerned about the possibility that Assad may use chemical weapons again.

“Just so there’s no confusion here, if the Syrian regime uses chemical weapons we will respond very strongly and they really ought to think about this a long time,” he added.

The Idlib region, a refuge for civilians and rebels displaced from other areas of Syria as well as powerful armed forces, was hit by a wave of air attacks and shelling this month, in a possible prelude to a full-scale government offensive.

In April, the Trump administration mustered a coalition of US, French and British forces to attack Syrian government facilities allegedly related to the production of chemical weapons after a poison gas attack killed dozens of people in Douma district.

Damascus, endorsed by Moscow, has denied using such weapons.

Under Trump, the US has sought to disengage from Syria, where the previous administration deployed some troops and gave limited support to rebel Kurdish forces over the objections of NATO partner Turkey.

Iran sanctions

Bolton also said that the Trump administration was not seeking to overthrow Iran’s leadership with its reimposition of sanctions on Tehran.

“Regime change in Iran is not American policy but what we want is massive change in the regime’s behaviour,” he said.

Bolton arrived in Israel on Sunday for three days of talks expected to focus mainly on Iran and its presence in Syria.

Israel and Syria share a border and Iran is backing Assad in his country’s civil war, along with Russia and Iranian-backed Lebanese Shia group Hezbollah.

Israeli Prime Minister Benjamin Netanyahu has pledged to prevent Iran from entrenching itself militarily in Syria, and a series of recent attacks that killed Iranians there has been attributed to Israel.

“Every time that Iran has brought missiles or other threatening weapons into Syria in recent months Israel has struck those targets,” Bolton said.

“I think that’s a legitimate act of self-defence on the part of Israel,” he added.

The Trump administration slapped back sanctions this month after withdrawing from the 2015 international nuclear deal with Iran, which Washington regards as insufficient for denying Tehran the means to make an atomic bomb and as a spur for its meddling in neighbouring Middle East countries.

The US turnaround outraged Iran, which has taken a defiant stance, and has rattled other world powers where some businesses have been debating whether to divest from Iran.

“Let me be clear, the reimposition of the sanctions, we think, is already having a significant effect on Iran’s economy and on, really, popular opinion inside Iran,” Bolton told Reuters in advance of Wednesday’s press conference.

“I think the effects, the economic effects certainly, are even stronger than we anticipated,” Bolton said.

“But Iranian activity in the region has continued to be belligerent: what they are doing in Iraq, what they are doing in Syria, what they are doing with Hezbollah in Lebanon, what they are doing in Yemen, what they have threatened to do in the Strait of Hormuz.”

The Iranian economy has been beset by high unemployment, inflation and a rial currency that has lost half its value since April. The reimposition of sanctions could make matters worse.

Thousands of Iranians have protested in recent weeks against sharp price rises of some food items, a lack of jobs and state corruption. The protests over the cost of living have often turned into anti-government rallies.

Commenting on the speech, Al Jazeera’s Harry Fawcett, reporting from Jerusalem, said: “It is clear that both he [Bolton] and Israeli prime minister and Israeli government are almost in lockstep in their view on the ‘Iran problem’, as Netanyahu continues to outline it.”

“Particularly in terms of the rejection of the Obama-era nuclear deal with Iran and its deleterious effects and the need for continued pressure for the sanctions the US has imposed to be shared and endorsed by the [other] countries,” he added.