FRANKFURT — A week before international peace talks on Syria are to begin in Switzerland, Western leaders and intelligence officials are reacting with alarm to what they say are the rising numbers of young people leaving Europe and the United States to wage jihad against President Bashar al-Assad.

The public comments, which coincided with an international donors conference on Wednesday for victims of Syria’s war, have highlighted the issue Western nations face. Concerns over the continuance of what they call Mr. Assad’s dictatorial rule must be weighed against the threat that the rebellion, which the West supports, is creating a new generation of jihadists.

Across Europe, intelligence officials, police officers, social workers and teachers have reported an increased push in recent months by Islamist radicals to recruit young Europeans to fight on the Syrian battlefield. Most are men, but some women have also been drawn to Syria with the prospect of helping establish an Islamic state, according to German officials and experts monitoring the trend.

American and European intelligence officials estimate that 1,200 young people have left to join Syria’s rebel groups, some of which have ties to Al Qaeda. On Tuesday, President François Hollande of France, in a news conference, said that French intelligence had counted 700 French citizens and foreigners who had headed to Syria from his country. “We must prevent them,” he said.