The war in Syria will "drag on" as long as it is part of a global conflict funded by others, President Bashar al-Assad said.

In an interview with AP in Damascus, the Syrian leader claimed the United States is "not genuine" about a ceasefire and "does not have the will" to deal with Islamic State.

"When you talk about it as part of a global conflict and a regional conflict, when you have many external factors that you don't control, it's going to drag on," Assad said.

"I believe that the United States is not genuine regarding having a cessation of violence in Syria."

"The United States doesn't have the will to work against al-Nusra or even ISIS because they believe that this is a card they can use for their own agenda," he added.

The president dismissed claims by Washington that an attack on a Red Crescent aid convoy, that killed 20 people, was carried out by a Russian-piloted aircraft loyal to his regime.

And he said that the US had intentionally hit Syrian troops in an attack Deir al Zour province which Washington had claimed was an accident.

It wasn't an accident by one airplane. It was four airplanes that kept attacking the position of the Syrian troops for nearly one hour, or a little bit more than one hour. You don't commit a mistake for more than one hour. Syrian President Bashar al-Assad

An attack on an aid convoy near Aleppo caused global outrage. Credit: Reuters

On the attack on a Red Crescent aid convoy, which killed 20 civilians including an aid worker, Assad denied responsibility adding it would "play into the hands" of terrorists.

"As a government, as officials, we are committed morally toward the Syrian people, morally, constitutionally, and legally, to help them in every aspect to have the basic needs for their livelihood," he said.

He said his enemies alone were to blame for nearly six years of devastation across the country and a mass humanitarian crisis.

Millions have fled Syria, some of them drowning at sea in the Mediterranean in a bid to seek refuge in Europe.

He said that civilians are not fleeing his country due to government forces but the embargo by the West on Syria.

"Many people, they flee not the war itself, but the consequences of the war, because they want to live, they want to have the basic needs for their livelihood, they don't have it," he said.

In recent months, Assad's forces have taken rebel strongholds in suburbs of the capital, bolstering security and reducing the threat of mortar shells.

Throughout the conflict, his forces have been accused of bombing hospitals and civilians and choking opposition cities.