Syrian President Bashar al-Assad said in an interview published on Thursday he expected his country's conflict to be long and difficult but vowed to defeat the insurgents battling to oust him and said he would not be driven from power.
Assad told the French magazine Paris Match nobody could predict when the war with Islamic State militants and other foes would end but said they had failed to win over the Syrian people, allowing his army to make advances.
Underscoring the challenge facing Assad, a monitoring group said at least 19 Syrian soldiers and militia men were killed when Islamic State militants attacked the Deir al-Zor airbase, one of the army's few remaining strongholds in eastern Syria.
"The Syrian army cannot be everywhere at once. Where it is not present, terrorists take the opportunity to cross borders and infiltrate in one area or another," Assad said.
"It is not about a war between two armies ...We are dealing with terrorist groups that infiltrate a town or village. So this war will be long and difficult," he added.
The text was also carried by Syrian state media on Thursday.
In extracts of the Paris Match interview published on Wednesday, Assad also branded U.S.-led air strikes in Syria since September as an "illegal intervention" that had made no difference in the fight against Islamic State.
Asked whether he saw his own departure from power as the solution, he said: "The state is like a ship: the captain does not escape in the storm. He does not quit the deck. If passengers need to leave, then he is the last to go."
The Britain-based Syrian Observatory for Human Rights said Islamic State had shelled the Deir al-Zor air base on Wednesday along with other areas still under government control.
Syrian state news said government forces had inflicted heavy losses on the Islamic State fighters in Deir al-Zor city.
Islamic State, which has seized swathes of territory in both Syria and Iraq this year, has been steadily consolidating its grip over Syria's oil-producing province of Deir al-Zor and now controls all but a few pockets.
Assad disputed a United Nations estimate that Syria's conflict had killed nearly 200,000 people since it erupted in 2011, saying figures in the media had been exaggerated.
He also rejected a suggestion that Syria had allowed Islamic State to flourish earlier in the war to wipe out other insurgents and accused the United States of creating conditions for the emergence of the group through its occupation of Iraq.
Islamic State is also fighting Syrian Kurds in the town of Kobani on the Turkish border. A second group of 150 Iraqi Kurdish peshmerga has entered Kobani from Turkey to replace a first group, peshmerga sources said on Thursday.
France's Foreign Minister Laurent Fabius dismissed Assad's comments in his interview on Thursday as "absurd", telling France 2 TV: "How can you imagine that somebody who caused 200,000 deaths can stay permanently at the head of his country?"