WASHINGTON — The United States is facing "heightened" threats of attacks from extremists, possibly the highest since the attacks of September 11, 2001, the head of US homeland security told lawmakers Wednesday.

"There is no question that we have made many important strides in securing our country from terrorism since 9/11, but the threat continues to evolve," Homeland Security Secretary Janet Napolitano told a congressional panel.

"And in some ways, the threat today may be at its most heightened state since the attacks nearly 10 years ago."

Napolitano, appearing before the House Homeland Security Committee, said that Al-Qaeda "still represents a threat to the United States, despite its diminished capabilities," and that the nation also faces "threats from a number of Al-Qaeda associates that share its violent extremist ideology."

She noted that there is "an increased emphasis on recruiting Americans and Westerners to carry out attacks."

"These groups are trying to recruit people to carry out attacks that have connections to the West, but who do not have strong ties to terrorist groups that could possibly tip off the intelligence community," she added.

"They are also encouraging individuals in the West to carry out their own small-scale attacks, which require less of the coordination and planning that could raise red flags and lead to an attack's disruption.

"This means that the threat has evolved in such a way that we have to add to our traditional counterterrorism strategies, which in the past have looked at the attack as coming from abroad," she said.

Napolitano, responding to questions, said the US has to be more alert in responding to threats from both outside and inside the country.

"We understand full well that Islamist-inspired, Al-Qaeda-inspired, however you want to call it, terrorism, be it coming from abroad or now being homegrown, is part and parcel of the security picture that we now have to deal with in the United States," she said.

Her comments came the same day a US man accused in a plot "to advance violent jihad" in other countries pleaded guilty to conspiracy to murder and other terrorism-related charges.

The Justice Department said 40-year-old Daniel Patrick Boyd, known by the name "Saifullah," pleaded guilty in federal court in North Carolina to charges of conspiracy to provide material support to terrorists and conspiracy to murder, kidnap, maim and injure persons in a foreign country.

It was the latest in a series of homegrown terrorism cases, and came just a week after an American woman who called herself "JihadJane" and surfed the Internet to recruit jihadists pleaded guilty to charges stemming from a plot to kill a Swedish cartoonist.

Colleen LaRose, 47, pleaded guilty on February 1 to terrorism, plotting to kill in a foreign country, lying to the FBI and attempted identity theft -- charges for which she could face life behind bars.

Last October, a Pakistani-American was sentenced to life in prison for his botched car bomb attempt in New York's famed Times Square, but in a chilling statement warned that jihadist war "has just begun."

Faisal Shahzad, a recently naturalized US citizen who lived in Connecticut and led what appeared to be a typical immigrant's life, pleaded guilty in June to the May 1 bombing attempt.