Al-Qaeda's deputy leader, Ayman al-Zawahiri, has said in a video the militant network will respond to attacks on Muslims in Lebanon and Gaza.
The video was broadcast by Arabic television station al-Jazeera.
Al-Qaeda could not remain silent in the face of a "Crusader war" and now saw "all the world as a battlefield open in front of us", he said.
Events in Lebanon and Gaza showed the importance of the battle in Afghanistan and Iraq, he added.
"The war with Israel is not about a treaty, a ceasefire agreement... It is rather a jihad for the sake of God until the religion of God is established. It is jihad for the liberation of Palestine, all of Palestine, as well as every land that was a home for Islam, from Andalusia to Iraq," he said.
"As they attack us everywhere, we will attack them everywhere. As they have joined forces to fight us, our nation will unite to fight them.
"The shells and rockets which are tearing the bodies of Muslims in Gaza and Lebanon are not purely Israeli. They are produced and financed by all the countries of the Crusader alliance.
"Therefore, all those who have taken part in the crime must pay the price. We cannot just watch these shells as they pour wrath on our brothers in Gaza and Lebanon and sit back in submission."
The statement is thought to be the first comment by al-Qaeda on Israel's offensive in Lebanon.