Al Qaeda leader Ayman al-Zawahri urged small-scale attacks inside the United States to "bleed America economically", adding he hoped eventually to see a more significant strike, according to the SITE monitoring service.
In an audio speech released online a day after the 12th anniversary of the 9/11 strikes, Zawahri said attacks "by one brother or a few of the brothers" would weaken the US economy by triggering big spending on security, SITE reported.
Western counter-terrorism chiefs have warned that radicalised "lone wolves" who might have had no direct contact with al Qaeda posed as great a risk as those who carried out complex plots like the 9/11 attacks.
"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," he said.
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 land "a large strike" on the United States, even if this took years of patience.
The Sept 11, 2011 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 extremists and their affiliates. Almost 3,000 people were killed in the attacks.
In his audio speech, Zawahri 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.