The threat of terrorism against the US homeland is in some aspects "at its most heightened state" since the 9/11 attacks, US Homeland Security Secretary Janet Napolitano has said.
"The terrorist threat facing our country has evolved significantly," she told members of Congress.
The US faced new threats by groups already inside the country, inspired by al-Qaeda, she said.
Ms Napolitano warned that attacks could be carried out with little warning.
Al-Qaeda and Taliban-linked individuals have attempted a number of recent attacks against the US, including a Nigerian who tried to blow up an aircraft with explosives hidden in his underwear, and an Afghan-born man who plotted to attack the New York subway.
State and local law enforcement officials are increasingly needed to combat terror, and the federal government must focus on supporting their efforts to secure communities, Ms Napolitano said in testimony to the House of Representatives Homeland Security Committee.
There is an increased reliance by foreign militant groups on recruiting individuals from the US, she added.
"As I have said before, we cannot guarantee that there will never be another terrorist attack, and we cannot seal our country under a glass dome," she said.
"However, we continue to do everything we can to reduce the risk of terrorism in our nation."
The country's counterterrorism response currently includes local "fusion centres" aimed at facilitating intelligence-sharing, a nationwide reporting initiative for suspicious acts and the "If you see something, say something" campaign created to "foster public vigilance", Ms Napolitano said.
As previously announced, the US is replacing the colour-coded terror alert system with a more useful one that can keep the public informed as much as possible, she added.
Al-Qaeda in the Arabian Peninsula and US-born Muslim cleric Anwar al-Awlaki, who has been labelled a "specially designated global terrorist" by his native country, pose the most significant threats to the US, Michael Leiter, director of the National Counterterrorism Center, told the committee.
Al-Qaeda was at its weakest point in the past 10 years, but it remained "a very determined enemy", he said.
Awlaki, believed to be hiding in southern Yemen, has posted sermons on the web thought to have inspired new recruits to Islamist militancy.
Ms Napolitano has previously said Awlaki is an example of a home-grown extremist spreading propaganda through the internet.
"Home grown radicalization is a growing threat, and one we cannot ignore," said Republican Representative Peter King, following Mr Leiter's comments.
Meanwhile, a US man, Daniel Patrick Boyd, pleaded guilty in a court on Wednesday in the state of North Carolina to plotting terrorist attacks.
More than 60 US citizens have been charged or convicted of terrorist acts or related crimes since 2009.