A group of EU countries signed an agreement to counter "foreign terrorist fighters" on Thursday, making it an international crime to travel abroad – or even plan a trip – to join terrorist groups.
"For the first time in international law, we have an instrument that criminalizes early preparations for acts of terror. It is the vital, missing piece of the jigsaw," said Thorbjorn Jagland, secretary general of the 47-member Council of Europe, which drew up the new law.
The amended Convention on the Prevention of Terrorism makes it a criminal offense to plan or raise money for a trip abroad, if the person intends to participate in any "terrorist offense, or the providing or receiving of training for terrorism."
Helping someone else to travel abroad to join a terrorist organization also becomes a criminal offense.
Seventeen EU members including Britain, France and Germany signed the new convention on Thursday.
"All of this shows our commitment to send a positive signal to all would-be terrorists: Europe is closing in, we are not waiting for you, we are coming for you," said Jagland.
The protocol was put together in a record seven weeks due to the threat posed by foreign fighters joining the ranks of jihadists in Syria and Iraq, he told assembled officials.
"Rarely has such a treaty received such unanimous support from the beginning."
The agreement must now be ratified by national parliaments, and will automatically come into force once six states have done so.
UN experts said this week that the Islamic State group is paying supporters up to $10,000 (8,800 euros) for each person that they recruit to fight in war-ravaged Syria and Iraq.
AFP contributed to this report.