Al-Qaeda chief Ayman al-Zawahiri has, in a video released in the run-up to the 15th anniversary of the 9/11 terrorist attacks, warned of more such incidents on US soil. Referring to the US, he has said that "as long as your crimes continue," such attacks would "be repeated a thousand times."

Al-Qaeda was responsible for four attacks on U.S. soil on Sept. 11, 2001, in which two hijacked passenger aircraft crashed into the twin towers of the World Trade Centre in New York, one into the Pentagon in Virginia, and one in Pennsylvania. It is believed that the terrorists aboard the fourth airplane wanted to crash it into the White House.

The crashing of the planes into the twin towers led to around 3,000 deaths, and marked a watershed moment in the world's view of terrorism and its fight against it. The U.S. has since toppled several oppressive regimes on foreign land in its fight against terror, and even hunted down Osama bin Laden, the man who masterminded the attacks.

It was expected that with bin Laden's killing on Pakistani soil and near a Pakistani Army base â€” despite Pakistan having constantly denied the world's most wanted terrorist's presence within the country â€” al-Qaeda would be severely weakened and eventually wither away. Now, with the emergence of this new video, and the new warnings issued by Zawahiri, it seems an old threat is back to haunt the United States.

However, experts are saying this could be just a ploy by the group to reassert the threat it once posed in the world, after it was superseded in threat levels by the resurgent Taliban â€” which has carried out a lot of deadly attacks in Pakistan â€” and the Islamic State group. The Isis is currently considered the deadliest terrorist group in the world after it claimed responsibility for a string of attacks in the Western world and on Western citizens in recent times.

In the latest video, Zawahiri is heard saying: "We mark in these days the passage of nearly 15 years since the blessed invasions in Washington, New York, and Pennsylvania."

He is also heard taking a swipe at what many believe is the Islamic State group, which is also known as Islamic State of Iraq and Syria (Isis) or Islamic State of Iraq and the Levant (Isil). Zawahiri says: "Real soldiers do not impose themselves on you as rulers without your acceptance and consultation."

Al-Qaeda had released a similar video on Sept. 11, 2011, the 10th anniversary of the ghastly terrorist attacks.