The death of a police officer during a counter-terrorism operation in Manchester has led to more questions about whether the asylum system is working effectively.

Tory MPs are pressing for tougher checks on immigrants arriving at UK ports of entry.

We need much more detailed screening of immigrants, involving the security services, with the emphasis on caution rather than compassion Patrick Mercer Tory MP

The government has responded by saying its overhaul of the asylum system has led to tighter control of potential immigrants.

Arrests in recent days under anti-terrorism legislation have been made across the UK, with many of those held Algerian or north African.

And those arrested following the operation in Manchester are also believed to be of North African descent.

Security experts believe some North African and Algerian terror suspects may have entered the UK as asylum seekers following a crackdown in France following attacks by Algerians in Paris in the 1990s.

At prime minister's questions on Wednesday, Tory leader Iain Duncan Smith said the incident and recent arrests of terror suspects showed the need to ensure borders were secure to "stop terrorists abusing the asylum procedure".

And former shadow home secretary Ann Widdecombe said there was a link between suspected terrorists and asylum seeking.

Tony Blair responded by saying recent changes to asylum laws had given immigration officials more power to turn people with criminal records away from ports.

And he criticised Tory MPs for "watering down" some of the government's proposals when the legislation was being debated last year.

Vetting

Earlier, shadow home secretary Oliver Letwin said there was "chaos" in the immigration system.

"Part of that appears to be a lack of vetting," he said.

And Conservative MP Patrick Mercer said the Manchester incident highlighted a need to be "much more robust" in terms of screening people seeking to enter the UK.

He told BBC News Online: "We are letting people in who are trying to damage the country.

"We need much more detailed screening of immigrants, involving the security services, with the emphasis on caution rather than compassion."

'Lost control'

Speaking on Radio Five Live last week, Mr Duncan Smith said arrests in Wood Green, London, had shown "to what degree we have to be vigilant about people who are likely to try and undermine our society and commit terrorist acts and be careful about who we let in".

He followed that up by saying that he believed the government had "lost control" of asylum policy.

He said there was no question that asylum seekers were coming to the UK in preference to other countries - and that those with a genuine fear for their lives if returned to their home countries should be welcomed.

But he went on: "Far too many people are finding enough money to abuse the system and come here as economic migrants masquerading as asylum seekers. What is clear is that some of those who come in here are criminals."

But Downing Street said in response to Mr Duncan Smith's comments that anyone applying for asylum in the UK is fingerprinted, while immigration officials have details of known terror suspects.

Reform

The prime minister's spokesman said last week that procedures were in place to seek to detect whether asylum seekers were involved in illegal activity or wanted in other countries.

The government's attempt to reform the asylum system became law last November.

The bill cleared the way for new citizenship, "accommodation centres" aimed at speeding up the processing of asylum-seekers and set new criteria for the acceptance of asylum claims.

It also included measures to allow the deportation of asylum seekers who have been convicted of criminal offences.

On Wednesday, a new European Union-wide database of asylum seekers' fingerprints began operating aimed at tackling people who apply for refugee status in more than one country.