The letter encouraged staff to "increase the level of alertness and to take special protection measures."

Security and checks were stepped up at the sprawling Palais des Nations, the original League of Nations, but work and meetings continued, U.N. spokeswoman Elena Ponomareva said.

The letter, a copy of which was obtained by Reuters, went further than a U.N. statement which referred merely to a "possible threat" against the organization.

"...the Swiss authorities have contacted us to inform of a possible terrorist attack against United Nations' premises in Geneva," said the letter.

"This attack would take place between 5th of October and the 10th of October: No information has been provided about the possible authors or the means to be used and no specific target has been mentioned," it said.

Eric Grandjean, a Geneva police spokesman, said that police were helping with patrols and checks at U.N. entrances at the request of U.N. security. He declined to give the numbers deployed, but said they were "adequate preventive measures."

It is the second time in a year that the U.N. in Geneva has been put on high alert.

The previous occasion coincided with a visit by U.N. Secretary-General Kofi Annan almost a year ago when a "credible security threat" was received via an unidentified U.N. member country.

Some 1,600 people work in the 1930s building, where the U.N. Human Rights Council is currently holding a three-week session to examine alleged violations -- including counter-terrorism measures taken in the wake of the Sept 11, 2001, attacks.

Annan had been due to be in Geneva this week but did not come as a planned summit in the Swiss city between the leaders of Gabon and Equatorial Guinea was canceled.

It said the situation was so grave that it had asked the government to provide police with armoured cars to protect officers in the estates, which are becoming no-go zones.

The number of attacks has risen by a third in two years. Police representatives told the newspaper Le Figaro that the "taboo" of attacking officers on patrol has been broken.

Instead, officers - especially those patrolling in pairs or small groups - faced attacks as soon as they tried to arrest locals.

Senior officers insisted that the problem was essentially criminal in nature, with crime bosses on the estates fighting back against tough tactics.

The interior minister, Nicolas Sarkozy, who is also the leading centre-Right candidate for the presidency, has sent heavily equipped units into areas with orders to regain control from drug smuggling gangs and other organised crime rings. Such aggressive raids were "disrupting the underground economy in the estates", one senior official told Le Figaro.

However, not all officers on the ground accept that essentially secular interpretation. Michel Thoomis, the secretary general of the hardline Action Police trade union, has written to Mr Sarkozy warning of an "intifada" on the estates and demanding that officers be given armoured cars in the most dangerous areas.

He said yesterday: "We are in a state of civil war, orchestrated by radical Islamists. This is not a question of urban violence any more, it is an intifada, with stones and Molotov cocktails. You no longer see two or three youths confronting police, you see whole tower blocks emptying into the streets to set their 'comrades' free when they are arrested."

He added: "We need armoured vehicles and water cannon. They are the only things that can disperse crowds of hundreds of people who are trying to kill police and burn their vehicles."

However, Gerard Demarcq, of the largest police unions, Alliance, dismissed talk of an "intifada" as representing the views of only a minority.

Mr Demarcq said that the increased attacks on officers were proof that the policy of "retaking territory" from criminal gangs was working.

Mayors in the worst affected suburbs, which saw weeks of riots and car-burning a year ago, have expressed fears of a vicious circle, as attacks by locals lead the police to harden their tactics, further increasing resentment.

As if to prove that point, there were angry reactions in the western Paris suburb of Les Mureaux following dawn raids in search of youths who attacked a police unit on Sunday. The raids led to one arrest. They followed clashes on Sunday night when scores of youths attacked seven officers who had tried to arrest a man for not wearing his seat belt while driving. That driver refused to stop, and later rammed a police car trying to block his path.

The mayor of Les Mureaux, Francois Garay, criticised aggressive police tactics that afterwards left "the people on the ground to pick up the pieces".