Sanctions that the United States reimposed on Iran have been more effective than expected, President Donald Trump's national security adviser said on Wednesday.

“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,” National Security Advisor John Bolton said during a visit to Israel.

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 neighboring Middle East countries.

“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 new sanctions have targeted Iran's trade in gold and other precious metals, purchases of US dollars and its car industry.

Trump has said the United States will issue another round of tougher sanctions in November that will target Iran's oil sales and banking sector.

Bolton said President Trump wants "maximum pressure on Iran, maximum pressure, and that is what is going on.”

He added: "There should not be any doubt that the United States wants this resolved peacefully, but we are fully prepared for any contingency that Iran creates.”

'Iran must leave Syria' 

Bolton also said the US is concerned about the presence of Iran and the withdrawal of Iranian forces is one prerequisite of resolving the Syrian conflict.

Bolton said Russia is "stuck" in Syria and looking for others to fund its post-war reconstruction, which is an opportunity to press for Iranian forces to quit the civil war-torn country.

"Our interests in Syria are to finish the destruction of the ISIS (ISIL) territorial caliphate and deal with the continuing threat of ISIS (ISIL) terrorism and to worry about the presence of Iranian militias and regular forces," he said.

"And those are the issues that keep us there."

"We're going see what we and others can agree in terms of resolving the conflict in Syria. But the one prerequisite there is the withdrawal of all Iranian forces back in Iran."