Britons heading to the Middle East to join the so-called Islamic State have travelled via Canada to avoid detection, the Foreign Secretary has said.
Philip Hammond told MPs that people are becoming "increasingly sophisticated" in their efforts to avoid detection.
During Foreign Office questions in the Commons, he was asked by Labour's Keith Vaz (Leicester East) what discussions he was having with international partners to stem the flow of British citizens.
Mr Hammond said: "It is one of the strands of work that the Isil (Islamic State) coalition is focused on. We have a number of working groups - one of them deals with foreign fighters.
"We have made considerable progress, particularly with our Turkish colleagues in ensuring we do everything possible to identify and intercept those seeking to reach Syria through Turkey.
"But people that are trying to take this journey are becoming increasingly sophisticated. I have seen reports recently of journeys which are routed via Canada to get to Turkey and then into Syria rather than going directly from the UK. So it is a continuing struggle."
Mr Vaz asked: "So far, 700 British citizens have travelled to Syria and Iraq and some to Yemen in order to fight for ISIL and al Qaida in the South Arabian peninsula.
"What discussions are you having with international partners to try and stop this from happening?"
Ministers were also asked to urge the Kurdish government to review the websites used to recruit foreign fighters after it was claimed a "quite vulnerable" man with autism from Newark had joined the Lions Of Rojava.
Tory MP Robert Jenrick (Newark) told Foreign Office minister Tobias Ellwood: "A young and quite vulnerable Newark man with autism has recently been recruited to fight with the Kurdish peshmerga through their foreign legion, the Lions Of Rojava, who recruit somewhat indiscriminately through Facebook and a website.
"While we all stand shoulder-to-shoulder with the brave peshmerga, will you urge the Kurdish government to exercise greater caution and particularly to review these websites?"
Mr Ellwood replied: "I am sorry to hear about the case of your constituent and if you'd like to meet afterwards I'd be delighted to take more details so we can look into it.
"I will be visiting the country shortly and I will speak in Erbil to see what can be done to make sure there is a better process to understanding who is coming into the country."