The United States should learn the lesson of Iraq and leave Syria, President Bashar al-Assad said in an interview, responding to U.S. President Donald Trump's description of him as an animal by saying "what you say is what you are".

In the interview with RT, the Russian state's international broadcaster, Assad raised the prospect of conflict with U.S. forces if they do not leave Syria. He vowed to recover territory where American troops have deployed, either through negotiations with Washington's Syrian allies or by force.

Assad, who is backed by Russia and Iran, appears militarily unassailable in the war that has killed an estimated half a million people, uprooted around 6 million people in the country, and driven another 5 million abroad as refugees.

After recovering swathes of territory, Assad now controls the biggest part of Syria. But tracts remain outside his control at the borders with Iraq, Jordan and Turkey.

That includes large parts of the north and east where U.S. special forces deployed during the fight against Islamic State, supporting the Kurdish-dominated Syrian Democratic Forces (SDF).

Assad said the government had "started now opening doors for negotiations" with the SDF, whose main component, the Kurdish YPG, has mostly avoided conflict with Damascus in the war.

"This is the first option. If not, we're going to resort to ... liberating those areas by force. We don't have any other options, with the Americans or without the Americans," he said. "The Americans should leave, somehow they're going to leave.

"They came to Iraq with no legal basis, and look what happened to them. They have to learn the lesson. Iraq is no exception, and Syria is no exception. People will not accept foreigners in this region anymore," he said.

The U.S.-led invasion of Iraq that toppled Saddam Hussein in 2003 was followed by an insurgency that lasted years.

Trump said in April he wanted to withdraw American troops from Syria relatively soon, but also voiced a desire to leave a "strong and lasting footprint".

U.S. Defense Secretary Jim Mattis said on April 30 the United States and its allies would not want to pull troops out of Syria before diplomats win the peace.

Trump called Assad an "animal" after a suspected poison gas attack on a rebel-held town near Damascus in April. Medical aid organisations said the attack killed dozens of people.

The attack triggered U.S., French and British missile strikes against what they called chemical weapons targets, the first coordinated Western strikes against Assad's government of the war. But the Western retaliation had no impact on the wider conflict, in which Assad's forces continued their advances.

In his interview, Assad reiterated the government's denial of blame for the chemical attack. Asked if he had a nickname for Trump similar to the "animal" comment, Assad replied: "This is not my language, so I cannot use similar language. This is his language. It represents him, and I think there is a well-known principle, that what you say is what you are."

AIR DEFENCES

Assad also sought in his interview to minimise the extent to which his government receives support from Iran.

Israel, which is deeply alarmed by Tehran's influence in Syria, said it destroyed dozens of Iranian military sites in Syria in May, after Iranian forces in Syria fired rockets at Israeli-held territory for the first time.

Assad said Iran's presence in Syria was limited to officers assisting the army. Apparently referring to the May 10 attack by Israel, Assad said: "We had tens of Syrian martyrs and wounded soldiers, not a single Iranian" casualty."

Asked if there was anything Syria could do to stop Israeli air strikes, he said the only option was to improve air defences, "and we are doing that". Syria's air defences were much stronger than before, thanks to Russia, he added.

Assad said the end of the war was getting closer with "every victory", but accused adversaries in the West and the region of trying to obstruct that and "hindering the political process".