Al-Qaeda leader Ayman al-Zawahiri called for jihadists to close ranks and take the battle to Western cities, especially in the United States, in an Internet audio message posted Sunday.

"I call on all mujahedeen (Islamist fighters) in districts of Iraq and Syria to collaborate and help each other," said Zawahiri, who opposes last year's declaration by the Islamic State group (IS) of a "caliphate" straddling the two Arab states.

Al-Qaeda's chief, who is in hiding, said the decision on the caliphate had been taken "in secret and without consultation".

He singled out five areas of cooperation: ending inter-jihadist clashes, a halt to campaigns calling for rivals to be eliminated, the creation of an independent religious tribunal to resolve differences, a general amnesty and collaboration in treating the wounded, sheltering refugees and stocking food supplies.

Such cooperation had become even more urgent because "the enemy has intensified its crusade against Islam".

To counter the campaign, Zawahiri called for Muslims "to take the war into the West's cities and against their installations, especially in the United States".

He urged jihadists living in Western countries "to identify their targets... and find the means to attack, avoiding spies and moving into action".

Al-Qaeda's Syria affiliate, Al-Nusra Front, allied with other Islamist rebel groups, has been locked in deadly clashes with IS for control of territory in the north and around Damascus.