There is no evidence that Isis is systematically using the refugee crisis to smuggle fighters into Europe or directly targeting asylum seekers for radicalisation, research has found.

The arrest of two refugees in connection with the attempted bombing on a London Underground train has reignited public debate, following four previous terror attacks in the UK this year.

Both men, an 18-year-old from Iraq and 21-year-old Syria, remain in custody after police were granted more time to question them over an attack that injured 30 people at Parsons Green station.

Isis claimed responsibility for the blast, saying it was carried out by “soldiers of the caliphate” and threatening further atrocities in the UK.

New research by the Institute for the Study of War (ISW) warned that the terrorist group was expanding its campaign in Europe as it loses territory in the so-called Islamic State.

The US-based think tank said Isis continues to plan, resource, and execute attacks from its remaining safe havens in Syria, Iraq and Libya.

This year has seen atrocities spread to the UK and Spain, as well as in Sweden and Finland, where the ISW said Isis was targeting “vulnerable populations”.

Timeline: European terror attacks since 2015

Asylum seekers launched two terror attacks in Germany last year, while the group used the refugee crisis as cover to smuggle the “supercell” of foreign fighters that carried out the Paris and Brussels attacks back into Europe using fake passports.

The ISW concluded that while refugees have conducted terror attacks in other countries, there was no evidence of “coordinated Isis involvement”.

Researchers warned that a larger threat came from the 2,500 remaining foreign fighters in Isis’ ranks in Syria and Iraq, who are able to form links with jihadi and criminal networks in their home countries even if they do not return.

The ISW warned that Isis would continue to organise and inspire terror attacks in Europe even while much of its physical caliphate is being wiped out.

“The anti-Isis coalition must also refocus on addressing the grievances and fears of vulnerable Sunni populations that make them vulnerable to Isis messaging,” the report concluded.

“The widespread perception that European states and the US are aligned with Iran and the Assad regime against Sunni populations in Syria and Iraq, as well as the rise of anti-Muslim sentiments and perceived anti-Muslim policies in Europe and the US will continue to fuel toleration of and limited but growing support for Isis attack operations in the West.

“Isis is waging a social and informational campaign to gain support among Western populations - Western states will not be able to kill and arrest their way out of this problem.”

Terror attack at Parson's Green – in pictures Show all 23 1 /23 Terror attack at Parson's Green – in pictures Terror attack at Parson's Green – in pictures Parsons Green explosion A bucket with flames and wires coming out of it was photographed in the carriage after the explosion – apparently the source of the blast AFP Terror attack at Parson's Green – in pictures Parsons Green explosion An explosion on a packed Tube train has injured a number of people in west London AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion The blast triggered a stampede as commuters panicked AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion Click through for more pictures from the scene Reuters Terror attack at Parson's Green – in pictures Parsons Green explosioN A police officer and a sniffer dog stand in the street near Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Kevin Coombs Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion Handout photo issued by James Treen of emergency services attending an incident at Parsons Green station in west London amid reports of an explosion. PRESS ASSOCIATION Photo. Issue date: Friday September 15, 2017. See PA story TRANSPORT Explosion. Photo credit should read: James Treen/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Terror attack at Parson's Green – in pictures Parsons Green explosion Police vehicles line the street near Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Kevin Coombs Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion A police officer stands outside Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Kevin Coombs Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion Armed British police officers stand on duty outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns. / AFP PHOTO / Daniel LEAL-OLIVAS / ===RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / HO / @RRIGS - NO MARKETING NO ADVERTISING CAMPAIGNS NO ARCHIVES - DISTRIBUTED AS A SERVICE TO CLIENTS FROM FROM ALTERNATIVE SOURCES, THEREFORE AFP IS NOT RESPONSIBLE FOR ANY DIGITAL ALTERATIONS TO THE PICTURE'S EDITORIAL CONTENT, DATE AND LOCATION WHICH CANNOT BE INDEPENDENTLY VERIFIED == / DANIEL LEAL-OLIVAS/AFP/Getty Images AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion Emergency services attend the scene following a blast on an underground train at Parsons Green tube station in West London, Britain September 15, 2017, in this image taken from social media. TWITTER / @ASolopovas/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. NO RESALES. NO ARCHIVES. MANDATORY CREDIT Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion Armed British police officers stand on duty outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns. / AFP PHOTO / Daniel LEAL-OLIVAS / ===RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / HO / @RRIGS - NO MARKETING NO ADVERTISING CAMPAIGNS NO ARCHIVES - DISTRIBUTED AS A SERVICE TO CLIENTS FROM FROM ALTERNATIVE SOURCES, THEREFORE AFP IS NOT RESPONSIBLE FOR ANY DIGITAL ALTERATIONS TO THE PICTURE'S EDITORIAL CONTENT, DATE AND LOCATION WHICH CANNOT BE INDEPENDENTLY VERIFIED == / DANIEL LEAL-OLIVAS/AFP/Getty Images AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion Armed British police officers stand on duty outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns. / AFP PHOTO / Daniel LEAL-OLIVAS / ===RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / HO / @RRIGS - NO MARKETING NO ADVERTISING CAMPAIGNS NO ARCHIVES - DISTRIBUTED AS A SERVICE TO CLIENTS FROM FROM ALTERNATIVE SOURCES, THEREFORE AFP IS NOT RESPONSIBLE FOR ANY DIGITAL ALTERATIONS TO THE PICTURE'S EDITORIAL CONTENT, DATE AND LOCATION WHICH CANNOT BE INDEPENDENTLY VERIFIED == / DANIEL LEAL-OLIVAS/AFP/Getty Images AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion Armed policemen stand by cordon near Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Hannah McKay Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion A woman reacts outside Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Kevin Coombs Reuters Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: Emergency services outside of Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Jack Taylor/Getty Images) Getty Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: A fireman walks past the police cordon at Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Jack Taylor/Getty Images) Getty Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: A tube train is seen stopped at Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Chris J Ratcliffe/Getty Images) Getty Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: Police officers stand by a cordon at Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Jack Taylor/Getty Images) Getty Terror attack at Parson's Green – in pictures Police forensics officers works alongside an underground tube train at a platform at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. "We are aware of an incident at Parsons Green tube station. Officers are in attendance," London's Metropolitan Police said on Twitter. / AFP PHOTO / Daniel LEAL-OLIVAS / (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images) AFP/Getty Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: Emergency serivces attend to the scene near Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Jack Taylor/Getty Images) Getty Terror attack at Parson's Green – in pictures Members of the emergency services work outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. British police are treating an incident on a London Underground train on Friday as an act of terrorism, saying "a number of people" had been injured. "Terrorist incident declared at Parsons Green Underground Station," police said in a statement. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images) AFP/Getty Terror attack at Parson's Green – in pictures Armed British police officers walk through the carriage of a London underground tube carriage at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images) AFP/Getty Terror attack at Parson's Green – in pictures Members of the emergency services and armed police officers work alongside an underground tube train at a platform at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. "We are aware of an incident at Parsons Green tube station. Officers are in attendance," London's Metropolitan Police said on Twitter. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images) AFP/Getty

The number of arrests for terror offences in the UK has risen almost 70 per cent in a year to a record high of 379, with the vast majority of suspects being British or dual nationals.

Ben Wallace, the security minister, told The Independent 19 terror plots have been foiled in the same period but warned that the tempo of attempted attacks was increasing.

Europol, the EU’s law enforcement agency, said there was a total of 142 terror attacks and attempts in just eight countries in 2016 – with more than half reported by Britain.

Its 2017 EU Terrorism Situation and Trend Report said Isis had exploited the flow of migrants to Europe to deploy terrorists for attacks and would continue the strategy.

But the report found there was “no concrete evidence that terrorist travellers systematically use those flows of refugees to enter Europe unnoticed”.

Europol previously said jihadis using the migrant route tended to be “expendable footmen”, while the most highly trained and dangerous operatives were likely to use false documents and more sophisticated methods.

“Given that it is in the interests of Isis to inflame the migration crisis to polarise the EU population and turn sections of it against those seeking asylum, there is a risk of some infiltration of refugee camps and other groups,” said a report issued in December.

“The extent of this is unknown, however, making the subject susceptible to exaggeration and exploitation especially by populist factions and extreme right-wing parties.”

Mohammad Daleel, a Syrian asylum-seeker, killed himself in a suicide bombing in the German city of Ansbach last year (AFP/Getty Images)

Europol said Isis was also seeking to exploit the crisis ideologically, with German authorities reporting that jihadis have made at least 300 attempts to recruit refugees attempting to enter Europe.

“A real and imminent danger is the possibility of elements of the (Sunni Muslim) Syrian refugee diaspora becoming vulnerable to radicalisation once in Europe and being specifically targeted by Islamic extremist recruiters,” a report warned.

“It is believed that a number of jihadists are travelling through Europe for this purpose.”

Europol also reported that refugees and ethnic minorities were facing increased violence throughout the EU, saying the vote for Brexit generated a spike in hate crime in the UK.

The agency is recruiting 200 counter-terror investigators to be deployed to migration hotspots in Greece and Italy to identify suspected terrorists and criminals in what Europol called “a second line of defence”.

The two refugees arrested in connection with the Parsons Green attack are believed to have entered the UK several years ago as children, being taken in by the same foster family in Surrey.

The 18-year-old Iraqi and 21-year-old Syrian remain in custody as searches continue at their former homes and a chicken shop where the older suspect worked.

The younger suspect was arrested on Saturday morning in Dover, where he was attempting to buy a ferry ticket to France.

The older man, named locally as Yahyah Farroukh, was detained by undercover police, including one disguised as a homeless man, outside a fast food restaurant in Hounslow later on the same day.