Syrian President Bashar al-Assad has made his first public admission of defeat to rebel fighters, stating "ups and downs" were normal in a war following a string of defeats in the country's north.

Speaking at an event to commemorate Syria's Martyrs' Day, Assad said the "setbacks" were part of war and did not mean the conflict was lost.

"Today we are fighting a war, not a battle. War is not one battle, but a series of many battles," he told supporters at a school in Damascus.

"So when setbacks occur, it is our duty as a society to boost the morale of the soldier and not wait for him to raise ours."

Assad urged his supporters to remain confident in the face of setbacks, warning against "despair at a loss here or there".

In the past few weeks, the Syrian government has faced a string of losses, particularly in northwestern Idlib province.

The "Army of Conquest" - a rebel coalition made up of the Nusra Front, Ahrar al-Sham, Jund al-Aqsa, Jaish al-Sunna, Liwa al-Haqq, Ajnad al-Sham, and Faynad al-Sham - seized Idlib's provincial capital, the strategic town of Jisr al-Shughur, and a military base in the region.

While Assad did not explicitly acknowledge his army's losses in Idlib, he paid tribute to regime forces that remain holed up in a hospital building in the now-rebel-held town of Jisr al-Shughur.

"The army will arrive soon to these heroes trapped in the Jisr al-Shughur hospital," he pledged.

The losses in the province, along with rebel advances in the south, have worried some in government-held areas and prompted speculation about the strength of the regime's forces.

Fighting continued on Wednesday between forces loyal to Assad and rebel fighters in Latakia, the heartland of Assad's minority Alawite community, as the Islamic State of Iraq and the Levant (ISIL) group launched an offensive on a major regime airport in Deir Ezzor city.

More than 220,000 people have been killed since March 2011, when anti-government demonstrations were met with a brutal crackdown by Assad's forces.