The United States faces immediate threats from lone-wolf terror attacks and in three years could see more mass casualties if al-Qaida rejuvenates after the American military leaves Afghanistan, terrorism experts said Tuesday at a Manhattan security conference.

The sobering assessments about the threat of a terror attack by individuals -- such as last year when four NYPD officers where attacked by a hatchet-wielding man in Queens -- were shared by two prominent counterterrorism experts speaking at the NYPD-sponsored conference at police headquarters.

"I would say the increase in moderate to small scale attacks in the West that we have seen since last summer by individual extremists reinforces our assessment that the most likely, most immediate threat to the homeland will in fact come from homegrown violent extremists," Nicholas Rasmussen, director of the National Counterterrorism Center, told hundreds of private and public security experts.

Former CIA deputy director Michael Morell agreed with Rasmussen about the danger of lone wolf strikes like the Paris attack in January that killed 17, including three terrorists, and the Denmark shooting this past weekend where the gunman and two others died.

"The lone wolf kind of attack, that threat to our homeland, the United States of America, has never been greater," said Morell, referring to al-Qaida, its offshoot group al-Qaida in the Arabian Peninsula, and the Islamic State, known as ISIS. Those groups use slick Internet campaigns to inspire individuals to attack the West, he said.

Rasmussen briefs President Barack Obama and other White House officials on terrorism. Morell was with President George W. Bush the moment he learned of the Sept. 11, 2001, attacks. He was with Obama in 2011 when Osama bin Laden was killed. The war against terror has undercut the ability of extremists to hit the United States in a catastrophic September 11-style attack, Rasmussen said, but he and Morell shared the view that continuing the pressure on terrorists groups was key to keeping them off balance.

When the United States leaves Afghanistan, Morell said, he feared the Taliban would gain ground and allow al-Qaida's return. There are currently about 10,800 U.S. military personnel in Afghanistan with a reduction to 5,500 planned by the end of the year. Obama has pledged to bring all troops home except for those guarding the U.S. Embassy in Kabul by the time he leaves office in 2017.

"If they [al-Qaida] go back across that border into Afghanistan and the pressure is taken off them and they have safe haven, they will develop the capability to attack us pre-9/11 style in, I will say, three years," Morell said.

With wire reports