Two arrests in Brussels brought to 13 the number of people arrested on Friday in Belgium, the UK and France as part of anti-terrorism investigations connected with the September 11 attacks in the US.

In Brussels, two men were charged on Friday night with conspiracy and attempting to cause explosions, after several kilos of chemicals - including sulphur and acetone - were found in a raid on an apartment in the Lemonnier district of the city. Police said it was clear a terrorist plot had been foiled, but that the prospective targets were unknown, and that no link had been established with Osama bin Laden, the presumed guiding hand behind the US attacks.

Also on Friday, seven men and women suspected of links with Islamic extremist movements were arrested at dawn in the Paris suburbs as part of an investigation launched the day before the attacks. British police arrested two men aged 27 and 29 and a 25-year-old woman in London. A third man was held in Birmingham.

"They were arrested in connection with the World Trade Center terrorist attack and are being questioned by anti-terrorist branch officers," Scotland Yard said.

The four have not yet been charged, and were arrested on the basis of FBI intelligence. A Whitehall official said: "Both the FBI and British security officials do not at this stage believe the UK end of the investigation is too significant."

The seven arrested in France are believed to have connections with Mr bin Laden, and to have knowledge of a plot involving a planned assault on the US embassy in Paris.

Another Franco-Algerian, Habib Zacarias Moussaoui, is already in police custody in the US. He has be en held on immigration charges but his attempt to pay cash to train on a commercial jet simulator raised suspicions that he was linked to the men who crashed the hijacked airliners. Despite France's increased domestic activity, the government and its Arab allies urged international caution on Friday. Hubert Vedrine, the French foreign minister, warned of the "devilish trap" apparently set by the perpetrators of the US attacks, saying he feared that an ill-focused US retaliation would anger Muslims around the world, leading to a dangerous "clash of civilisations" between the west and Islam and prompting a counterattack by the terrorists.

French officials said they were taking seriously the possibility that Mr bin Laden or his associates would attempt to use biological or chemical weapons to kill people in a crowded western city in the next stage of their war against the west. In the UK, officials said that Tony Blair, the prime minister, was determined to support the US in any likely military assault on terrorist targets.

But they said there were some concerns about terrorist retaliation that could include the use of biological or chemical weapons.

In the US, Virginia Governor James Gilmore, who chairs a national advisory panel on domestic responses to terrorism, said this week that still broader measures to protect the security of the nation's transport system might be necessary.

Mr Gilmore's panel, which will release its final report next month, is also moving towards recommending the stockpiling of serums to help those infected in a biological attack with anthrax or other deadly organisms.