"The enemy - while weakened and under great pressure - is still capable of global reach, still possesses the determination to kill more Americans, and still trying to do so with increasingly powerful weapons," Defence Secretary Donald Rumsfeld told the National Press Club in Washington on Thursday.
The US strategy, he said, includes doing everything possible to prevent the enemy from gaining weapons of mass destruction, improving homeland defence and intelligence gathering and helping friendly nations become better able to fight the terrorists in their own countries.
"Because they lurk in shadows, without visible armies, and are willing to wait long periods between attacks, there is a tendency to underestimate the threat they pose," said Rumsfeld.
He said there are no fewer than 18 organisations, loosely connected with al-Qaida, conducting terrorist attacks.
"They will either succeed in changing our way of life, or we will succeed in changing theirs," he said.
Rumsfeld's remarks echoed comments from US intelligence chief John Negroponte, who told the US Senate Intelligence Committee that al-Qaida remains the "top concern" for US security.
"Because they lurk in shadows, without visible armies, and are willing to wait long periods between attacks, there is a tendency to underestimate the threat they pose"
US Defence Secretary
Also speaking on Thursday, Negroponte told senators that while states such as Iran and North Korea were of "highest concern" because of their suspected nuclear weapons programmes, the al-Qaida network remained the biggest threat because of its continued efforts to carry out "high-impact attacks" on Americans.
"Attacking the United States homeland, United States interests overseas and United States allies in that order are al-Qaida's top operational priorities," Negroponte said.
He added that al-Qaida was more likely to use conventional weapons in any attack on the United States, but it remains interested in acquiring chemical, biological, nuclear and radiological weapons.
The warnings come as the Pentagon is preparing to release a broad four-year defence review calling for more spending on special operations forces, cuts in Air Force personnel, and a restructuring of the Army and reserve forces.
Rumsfeld's speech also touched on the idea that Americans must be braced for a long war on terror, a theme that both Rumsfeld and President George Bush have pressed in recent days.
He also repeated the administration's often-stated warning that the only way terrorists win is if the US loses its will to continue the fight in Iraq.
President Bush has faced a growing domestic uneasiness with the Iraq war, which is costing more than $4 billion a month and has left more than 2240 US service members dead.