Media playback is unsupported on your device Media caption A video emerged last week of a British man who is thought to have carried out a suicide bombing in Syria

The "security concern" for the UK posed by individuals who have trained and fought in Syria is "a big problem" for MI5 and the police, the immigration minister has said.

James Brokenshire told the BBC "a significant and growing proportion" of their resources was spent on the issue.

He said it was right to be "vigilant" about travel between the UK and Syria.

According to the Sunday Times, security services are "closely monitoring" 250 British-based jihadis linked to Syria.

The authorities are concerned that such people may be radicalised and militarised - and urged by those whom they come into contact with in Syria to turn their attentions away from the Syrian regime and instead attack targets in the West.

Militants with suspected links to al-Qaeda have been heading to war-torn Syria from many other countries since fighting broke out in 2011.

Media playback is unsupported on your device Media caption James Brokenshire: "The growing proportion of the security services work is linked to Syria in some way"

Last week, a video was posted online showing British man Abdul Waheed Majid, who is thought to have carried out a suicide bombing in the city of Aleppo.

The 41-year-old, from Crawley, West Sussex, was believed to have bombed a jail on 6 February.

'Jihadist destination'

In an interview with Radio 4's The World This Weekend, Mr Brokenshire said he believed the "security concern" linked to Syria was "likely to be with us for the foreseeable future".

"A significant proportion and a growing proportion of the security services work is linked to Syria in some way," he said.

"This is a big problem that the security services and the police are actively focused on.

Our biggest worry is when they return they are radicalised Sir Bernard Hogan-Howe, Metropolitan Police Commissioner

"It's why they are vigilant, why they are taking the steps that they are around the border and monitoring travel to and from Syria in the way that they are."

The immigration and security minister said Syria had become "the number one jihadist destination in the world" and the number of Britons thought to have travelled there to fight so far was in the "low hundreds".

Sir Peter Fahy, who leads the Association of Chief Police Officers "Prevent" strategy on counter-terrorism, told the BBC last month those returning from Syria would be stopped and could be arrested and charged.

Even those who were not arrested would be put on programmes, he said, which saw police work with local agencies such as schools and youth organisations, "essentially to make sure these people haven't been affected and try and make sure they're not a threat to this country".

Mr Brokenshire said he recognised "that not everyone who has been to Syria and is travelling back is involved in terrorism", but added: "Clearly the message is: 'People shouldn't be travelling.'

'Desire to help'

"The situation on the ground means that people who might think that they are going for genuine humanitarian purposes may get involved in situations they simply had not contemplated.

"It's very fluid and people may become involved in terrorist organisations who are killing civilians. Indeed, they may be radicalised when they are out there.

"When you look at the appalling situation in the region... I certainly recognise the absolute desire for people to want to help. But it's helping in the right way and the right way is by providing the financial support to organisations, recognised organisations, that are able to deliver."

The Sunday Times says the number of individuals being monitored by MI5 and the police is much higher than previously reported, underlining "the growing danger posed by 'extremist tourists'".

Throughout January, 16 people were arrested on suspicion of terror offences after travelling between Syria and the UK - that compares with 24 in the whole of 2013.