"In September 2014 less than 900 foreign terrorist fighters had been identified by Interpol. Today, in less than a year, more than 4,000 profiles are available in our database," Stock said.
Stock was speaking at the opening of a meeting in the northeastern Spanish city of police forces from around the world with Interpol which aims to boost cooperation in the fight against terrorism.
Stock said nations needed to boost their information sharing and improve access to the data they have for organisations like Interpol if they wanted to curb the flow of foreigners to jihadist groups.
"We must remember that while information is increasingly being shared across borders, it is much slower than it takes for foreign terrorists to recruit and travel to and from conflict zones," he said.
"A gap still exists between the number of foreign terrorist fighters we have identified and those estimated to have reached conflict zones. We need to work to close this gap," he added.
A recent UN report showed an increase in the number of foreign fighters from last year, with more than 25,000 foreign jihadists from more than 100 countries now involved in armed conflicts.
A large number of fighters were travelling from Tunisia, Morocco, France and Russia, but the report cited new flows of jihadists from the Maldives, Finland, Trinidad and Tobago, as well as from some countries in Sub-Saharan Africa.