WASHINGTON (AFP) - The United States on Tuesday denied that it was coordinating air strikes with the Syrian regime against Islamic militants and renewed its insistence that President Bashar al-Assad must step down.

In an interview with the BBC, Assad said Damascus was being informed about the US-led coalition air strikes against the Islamic State group through messages conveyed via a third party.

Assad said the communication was through third parties, including neighbouring Iraq, where Washington and Western allies are also carrying out strikes against ISIS (Islamic State in Iraq and Syria).

Before the strikes were launched in Syria in September, Washington did "inform the Syrian regime directly of our intent to take action through our ambassador to the United Nations", said State Department spokeswoman Jan Psaki, repeating information given at the time.

"We did not request the regime's permission. We did not coordinate our actions with the Syrian government," she said. "We did not provide advance notifications to the Syrians at a military level, nor give any indication of our timing on specific targets."

Washington had also warned Damascus "not to engage US aircraft".

Psaki refused to lay out any conversations with the Iraqi government and said the US did not pass on its plans every time, although she dodged a question about whether Damascus had only been informed once, in September.

The US position remains that "Assad has lost all legitimacy and must go .. There cannot be a stable inclusive Syria under Assad's leadership", Psaki added.

She also insisted that his interview with the BBC had to be "taken with a grain of salt."

In the same interview, "he denied the use of barrel bombs, chlorine and also the indiscriminate killing of his own people", Psaki said.