The amendment to the Council of Europe Convention on the Prevention of Terrorism aims to combat the problem of European citizens travelling to join terror groups on foreign soil.
It was opened for signature in Riga on Thursday and signed for Switzerland by Stephan Husy, Swiss special envoy for the international fight against terrorism.
The protocol outlaws travelling abroad for the purpose of terrorism, receiving training for terrorism, as well as the financing, organization and facilitation of such journeys.
France, Germany, the UK and Spain were among the other nations signing the amendment, which was prepared in a record time of seven weeks in view of the serious threat posed by foreign fighters, said Thorbjørn Jagland, secretary general of the Council of Europe, at the ceremony.
“For the first time in international law, we have an instrument that criminalises early preparations for acts of terror,” he said.
“The Additional Protocol to the Convention of Terrorism sets out how we can jointly take on foreign terrorist fighters within the rule of law and human rights. It is the vital, missing piece of the jigsaw.”
Now signed, the protocol must be ratified by the national parliaments of at least six of the 47 members of the Council of Europe.
Switzerland has been a member of the Council of Europe, which works to promote human rights, democracy and rule of law, since 1963.
Combating the rising number of Europeans going to fight in Syria and Iraq, as well as the radicalization of youths, is a “priority” for the Swiss government, foreign minister Didier Burkhalter said earlier this year.
In April a 25-year-old Swiss man was arrested at Zurich airport on suspicion of trying to travel to Syria to join jihadists.
In August Swiss media reported that teachers in Winterthur were being trained to spot the signs of radicalization among their students, after at least five young people from the area travelled to join Isis.
According to UN experts, Isis pays $10,000 to every recruit to the jihadist cause.