WASHINGTON - The United States is raising security alert around the country as it gears up for the 10th anniversary of the September 11 terror attacks, though White House insisted on Wednesday there is no specific terror threat.

As precaution, security has been tightened at US airports, mass transit stations, major government buildings and even athletic events, and will remain so in the next month, as the nation prepares for the anniversary of the 9/11 terror attacks, which killed nearly 3,000 people in New York, the Pentagon and Shanksville, Pennsylvania, in 2001.

The US government has planned a series of official memorial ceremonies to be held in New York, Pentagon, Washington D.C. and Shanksville ahead of and on September 11, to commemorate the victims.

State and local law enforcement agencies have been receiving a daily briefing on possible terror threats and ways to increase security in local communities from the Federal Bureau of Investigation (FBI) and Department of Homeland Security (DHS), US officials were quoted by US media as saying on Wednesday.

But John Brennan, the homeland security and counterterrorism adviser to President Barack Obama, insisted on Wednesday that the government had received no specific or credible terror threat that targets the 9/11 anniversary.

Noting that US security agencies are doing better in sharing intelligence than 10 years ago, Brennan said the FBI has been doing a good job in tracking down individuals with extremist ideology that could lead them to launch terror attacks.

"While there is currently no specific or credible threat, appropriate and prudent security measures are ready to detect and prevent plots against the United States should they emerge," said DHS spokesman Matt Chandler.

Currently, US security authorities are more worried about possible "lone-wolf" attacks by a single terrorist or terror- minded individual, because they are more difficult to discover than terror organizations.

Such worries were intensified in July, when a white supremacist killed 69 people in Norway in a single attack carried out under the name of saving Norway from Muslims and multiculturalism.      

US President Barack Obama issued a warning earlier this month that the threat of a "lone-wolf" terrorist attack was "particularly troublesome,"  because it could equally lead to wide- scale massacres of the sort as seen in Norway.