WASHINGTON (AFP) – The number of attempted attacks against the United States in the past nine months has "surpassed the number of attempts during any other previous one-year period," according to a Department of Homeland Security memo, CNN said.

The DHS assessment dated May 21, and reported on the CNN website late Wednesday, said extremist groups were expected to attempt more attacks inside the United States with "increased frequency."

The unclassified memo prepared for US law enforcement agencies urged authorities to "operate under the premise that other operatives are in the country and could advance plotting with little or no warning."

Highlighting recent cases of militants living in the country for years, as they radicalized, plotted and attempted to carry out attacks, the memo also warns of a reduced ability to track them -- such as Pakistani-American Faisal Shahzad, suspected in the attempted Times Square bombing earlier this month.

Shorter periods of training overseas "compared to lengthier training cycles for earlier operations," leads to problems detecting their activities, DHS said, according to the CNN report.

Other trends indicate plotters are also looking for "smaller, more achievable attacks against easily accessible targets," said the memo.

The report comes as President Barack Obama's administration makes the threat of homegrown extremism an integral part of US security policy.

In a new national security strategy to be released Thursday, the administration is likely to focus attention on the threat posed by homegrown, radical extremists following pressure on Al-Qaeda limiting the group's ability to move, raise funds, recruit and carry out attacks.

The network was now relying on poorly trained "foot soldiers" who might be able to slip past US defenses because they do not fit the conventional profile of a terrorist, said John Brennan, deputy national security advisor for counter-terrorism and homeland security, in a speech Wednesday.