Intelligence officials have intercepted a credible terror plot against Britain and France, raising security fears at the Eiffel Tower, but failing to raise the overall threat level in either country.

The Eiffel Tower was briefly evacuated on Tuesday evening after officials received a bomb threat called in from a telephone booth. It was the second such alert at the monument in two weeks.

The warning came as French officials were put on alert for possible terror attacks. British officials, too, have been aware of a possible attack but the terror threat warning has not changed from "severe".

"There have been a succession of terror operations we've been dealing with over recent weeks but one to two that have preoccupied us," said one British government official. "Still, it hasn't been to the degree that we have raised the threat level."

Another British official, who spoke on the same terms, would not confirm the plot was "al Qaida inspired" but said there was an "Islamist connection" and that the plots were in an early stage. No other details were given.

Since the September 11 terror attacks in the United States nine years ago, the terror group has moved outside of Afghanistan and Pakistan to other countries such as Somalia and Yemen.

German officials denied they had intercepted threats, saying there had been no change to their threat level.

French police on Tuesday closed off the surroundings of the Eiffel Tower, France's most visited monument. Officers pulled red-and-white police tape across a bridge leading over the Seine River to the monument. Officers stood guard.

Bomb experts combed through the 324-metre (1,063-ft) tower and found nothing unusual, the Paris police headquarters said. Tourists were let back inside about two hours after the structure was emptied.

Police responded to a similar false alert at the tower on September 14, also following a phone threat. On Monday, the bustling Saint Lazare train station in Paris was briefly evacuated and searched.