Syrian President Bashar al-Assad says US president-elect Donald Trump would be a "natural ally" against terrorism if he could overcome "countervailing forces" in the US administration.

"We don't have a lot of expectations because the American administration is not only about the president; it's about different powers within this administration," the Syrian leader told Portuguese television station RTP in his first comments on the outcome of the US election.

Asked about Trump's remarks on being ready to fight the Islamic State extremist group, al-Assad, who spoke in English during the interview broadcast late yesterday, said: "This is promising but can he deliver?"

"If he is going to fight the terrorists, of course we are going to be (an) ally, natural ally in that regard with the Russian(s), with the Iranian(s), with many other countries who wanted to defeat the terrorists."

Under outgoing President Barack Obama, the US has urged al-Assad to leave power and has supported the Syrian political opposition as well as giving strictly limited aid to some rebel groups.

The main US military involvement is in support of leftist Kurdish forces who are fighting the Islamic State jihadist organisation. The Kurds recently launched what they say is a campaign to capture al-Raqqa, the jihadists' de facto Syrian capital.

Al-Assad was less positive about US backing for the Kurds, whom both government and opposition accuse of seeking to divide the country.

"Any cooperation that doesn't go through the Syrian government is not legal," he told RTP.

Trump has suggested his foreign policy will be less hostile to al-Assad's government, whose forces are concentrating on fighting the mainly Islamist rebels but are also battling Islamic State in some areas.

The US president-elect told the Wall Street Journal on Friday: "My attitude was you're fighting Syria, Syria is fighting IS (Islamic State), and you have to get rid of IS... Now we're backing rebels against Syria, and we have no idea who these people are."

He had also previously said that fighting Islamic State was more important to him than removing al-Assad from power, suggesting a shift in American policy.

Speculation that Trump would cut back on the Obama administration's limited support for the Syrian opposition has been heightened by his warm words for al-Assad's key international backer, Russian President Vladimir Putin.

Trump has repeatedly refused to reveal anything about his strategy for defeating Islamic State, saying he did not want to give the terrorist group the chance to prepare for it.

The interview came as Russia upped its military involvement in Syria, launching air raids on opposition strongholds in the north-west from its Admiral Kuznetsov aircraft carrier which recently arrived in the eastern Mediterranean.

The rebel-held eastern half of Aleppo also saw its first air strikes in some three weeks, killing six people, according to the Britain-based Syrian Observatory for Human Rights.

An estimated 250,000 to 300,000 people are under siege in eastern Aleppo, where residents report declining or damaged supplies of necessities including food, water, electricity and medicine.

The Syrian conflict began in 2011 with peaceful protests against al-Assad's regime.

After a brutal crackdown by security forces, it descended into a four-sided civil war that has driven almost 5 million people from the country and left another 6.5 million internally displaced, according to UN figures.

- dpa