Syrian President Bashar al-Assad is "deluded" if he thinks there is a military solution to the war in Syria, nearly five years into a brutal conflict that has killed more than 260,000 people, a US official has said.

The defiant Syrian leader vowed to recapture Syria as a whole and keep "fighting terrorism".

"He's deluded if he thinks that there's a military solution to the conflict in Syria," deputy State Department spokesman Mark Toner told reporters. 

"All we're looking at, if the Syrian regime continues the fighting, is more bloodshed, more hardship and, frankly, a greater hardening of positions on either side."

The conflict has fuelled the meteoric rise of the self-proclaimed Islamic State group, which controls large swathes of the country.

Rebel forces also hold significant territory.

Assad spoke hours before world powers agreed on an ambitious plan to cease hostilities in Syria within a week, but doubts soon emerged over its viability, especially because it did not include IS or al-Qaeda's local branch.

"The proof is in the pudding," Mr Toner cautioned, in reference to the deal. 

"We need to see action on the ground on the part of those parties, and that includes the regime. That includes the opposition. They need to stop the fighting, and then we can determine who is part of this process and who is not."

Meanwhile, there are calls for Russia to halt its air strikes in Syria, in order for a deal aimed at ending the country's civil war to be successful. 

The UN said it hopes to begin delivering aid to remote areas of Syria today.

US Secretary of State John Kerry said the agreement on a "cessation of hostilities" is to take effect in a week's time. 

However, Russian Foreign Minister Sergei Lavrov has said Moscow will not stop the air assaults in the country.

The Turkish military has hit targets of the Syrian Kurdish Democratic Union Party (PYD) and the Syrian regime in two separate incidents in response to incoming fire, the state-run Anatolia news agency reported this evening.

In line with the rules of engagement, the armed forces shelled targets of the PYD around the town of Azaz in Aleppo province, Anatolia said, quoting a military source.

The army also responded to Syrian regime fire on a Turkish military guard post in Turkey's southern Hatay region, it added.

There were no further details on the nature of the Turkish strikes but they likely involved artillery fire from tanks.

Prime Minister Ahmet Davutoglu also appeared to confirm the strikes against the PYD, without giving precise details.

"Under the framework of the rules of engagement, we responded to forces in Azaz and around that were posing a threat," he said.

Apparently referring to the PYD, he called these forces "a terror group which is a branch of the Syrian regime, collaborationist and is complicit in Russian strikes against civilians". 

The United States has pressed Turkey to halt the military strikes.

"We are concerned about the situation north of Aleppo and are working to de-escalate tensions on all sides," US State Department spokesman John Kirby said in a statement.