By Nick Childs BBC Pentagon correspondent

The United States is stepping up its security for the first Independence Day since the 11 September attacks on the World Trade Center and the Pentagon.

There is widespread public anxiety - but there is no specific threat, the authorities say, and the national alert status is unchanged, at "yellow" or elevated alert.

Security around Capitol Hill has been beefed up

But, across the country, the authorities are tightening their security precautions.

US fighters will take to the air again on combat patrols over key American cities and randomly across the country.

There will be more FBI and police on the streets.

And the White House Office of Homeland Security has set up a special co-ordination centre to monitor more than 2,000 medium and large events throughout the United States.

Official message

While there has been no evidence of a direct threat, there have been weeks of speculation that America's Independence Day would present too tempting a target to miss.

All the precautions may themselves put people off celebrating.

The message from the White House is that they should enjoy the holiday, and let the authorities do their worrying for them.

But Americans may have to put up with some added inconvenience as a result of the added security.

A particular focus of Fourth of July celebrations is the National Mall around the Washington Monument, where hundreds of thousands of people usually gather to watch a spectacular fireworks display.

This year there are more surveillance cameras, extra security fencing, more screening and searches of visitors, and 2,000 uniformed police officers to oversee it all.