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The United States will refuse any additional reconstruction assistance to war-torn Syria so long as Iranian troops are present in the country, Secretary of State Mike Pompeo said on Wednesday.

Pompeo vowed to press forward with President Donald Trump's push to isolate Iran, boasting of imposing "some of the harshest sanctions in history."

"The onus for expelling Iran from the country falls on the Syrian government, which bears responsibility for its presence there," Pompeo told the Jewish Institute for National Security of America.

"If Syria doesn't ensure the total withdrawal of Iranian-backed troops, it will not receive one single dollar from the United States for reconstruction," Pompeo said.

Pompeo's remarks come as the Trump administration shifts its reasoning for US involvement in Syria's brutal civil war, which a war monitor says has killed close to 365,000 people since 2011.

The United States has some 2,000 troops in Syria, mainly training and advising rebels, after former president Barack Obama authorised the mission to defeat the Islamic State extremist group, or IS.

Pompeo said that fighting IS "continues to be a top priority" but listed rolling back Iran as another. 

National security adviser, John Bolton, said last month that US troops would stay "as long as Iranian troops are outside Iranian borders."

Iran and Russia are the primary supporters of Syrian President Bashar al-Assad in his bloody campaign against rebels and IS.

Pompeo did not make similar demands for a withdrawal by Russia, which has long considered Syria a major ally.

Trump in August already pulled out of Syria's immediate reconstruction, suspending $230 million after pledges by Gulf Arab allies.

Iran also backs Lebanese Shia movement Hizballah, which has thousands of fighters in Syria, backing Assad's forces.

After initially downplaying its military presence in Syria, Tehran now claims its troops are there at the request of the Syrian government.

Iran President Hassan Rouhani has also rejected the Trump administration's warnings regarding Syria.

Relations between Iran and the US have hit a new low, with Washington withdrawing earlier this year from a nuclear deal with Tehran.

Iran has been accused of being behind unrest in Yemen, Iraq, Bahrain, Saudi Arabia and Lebanon.

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