The leader of al-Qaeda has urged his followers to strike inside the US to "bleed America economically", adding that the United States can be defeated by targeting its economy, according to a message posted on a website frequently used by the group.

In an audio speech released online two days after the 12th anniversary of the 9/11 strikes, Ayman al-Zawahiri said that America is not a "mythic power" and that the mujahedeen - Islamic holy warriors - can defeat it with attacks "on its own soil".

"We should bleed America economically by provoking it to continue in its massive expenditure on its security, for the weak point of America is its economy, which has already begun to stagger due to the military and security expenditure," al-Zawahiri, the successor to Osama bin Laden, said.

The message's authenticity could not be independently confirmed.

Keeping America in such a state of tension and anticipation only required a few disparate attacks "here and there", he said.

"As we defeated it in the gang warfare in Somalia, Yemen, Iraq and Afghanistan, so we should follow it with ...war on its own land. These disparate strikes can be done by one brother or a few of the brothers."

At the same time, Muslims should seize any opportunity to stage small attacks or a "big strike" against the United States, even if this took years of patience, he said.

Enemies of Islam

The September 11, 2001 attacks, in which hijacked airliners were flown into New York's World Trade Center, the Pentagon in Washington and a Pennsylvania field, triggered a global fight against al-Qaeda members and their affiliates.

Almost 3,000 people were killed in the attacks.

In his audio speech, al-Zawahiri - who is believed to be hiding in the Pakistan-Afghanistan border region - said Muslims should refuse to buy goods from America and its allies, as such spending only helped to fund US military action in Muslim lands.

He added that Muslims should abandon the US dollar and replace it with the currency of nations that did not attack Muslims.

Al-Zawahiri spoke approvingly of one of the worst attacks on the US soil since September 11, 2001, the bombing of the Boston Marathon in April, which US authorities say was carried out by two ethnic Chechen Muslim brothers. The attack killed three people and injured 264.

On Syria's civil war, al-Zawahri addressed al-Qaeda-linked fighters - including many foreign fighters - who have taken an increasingly prominent role in the fight against President Bashar Assad's regime.

Their rise has caused tensions with more moderate Syrian rebel factions, even escalating to violence and turf battles.

America wants to use "the Muslim people as a means to topple the pro-Iran Baathist regime and install a secular government and peaceful to Israel", al-Zawahri said.

It "will try to push the mujahedeen to compromise with the secular factions and the enemies of Islam".

"I warn my brothers in Syria against any compromise with those factions. They have to learn the lesson of Egypt," al-Zawahri said.