Al Qaeda has returned to its longtime base of operations in southern Afghanistan and is plotting new attacks against the West, fifteen years after being overrun by U.S.-led NATO forces following the 9/11 attacks, according to a published report.

Britain's Daily Telegraph, citing Afghan security officials, reported Monday that Al Qaeda cells have moved back into southern Afghanistan following the withdrawal of most U.S. and allied troops in 2014. The report claimed that most of the cells are operating around the southern Afghan city of Kandahar, the spiritual home of the Taliban.

The Telegraph reports that security officials believe that Al Qaeda's return to Afghanistan is the latest move to re-establish its strength after the killing of Usama bin Laden by Navy SEALs in 2011 and the rise of ISIS, a former Al Qaeda splinter group.

The report comes days after a U.S. drone strike killed Afghan Taliban leader Mullah Mohammed Akhtar Mansour. His death was hailed by Afghan leaders as a game-changer in efforts to end the long insurgent war plaguing the country.

President Obama called Mansour "the leader of an organization that has continued to plot against and unleash attacks on American and Coalition forces, to wage war against the Afghan people, and align itself with extremist groups like Al Qaeda"

In a statement, Obama called on the Taliban to "seize the opportunity to pursue the only real path for ending this long conflict - joining the Afghan government in a reconciliation process that leads to lasting peace and stability."

After the fall of the Taliban in 2001, Al Qaeda operatives retreated across the border into remote areas of northwest Pakistan, where local tribal leaders hold more sway than the government in Islamabad.

"Al Qaeda has been desperate to get back into Afghanistan ever since they were ejected after 9/11," Col. Richard Kemp, a former commander of British troops in Afghanistan, told the paper. "Now they are back they will be desperate to launch a high profile attack against the West."

In addition to its numbers in Afghanistan, Al Qaeda is believed to have around 150 terrorists based in Syria and an unknown number based in Yemen, where an ongoing civil war between Iran-backed rebels and the Saudi-backed government has raged for 14 months.

The U.S. currently maintains a force of approximately 9,800 troops in Afghanistan. A further drawdown to 5,500 troops is set for late this year or early 2017.

The Associated Press contributed to this report.