Sweden has stepped up efforts to deport illegal immigrants after a failed asylum-seeker killed five people in the Stockholm truck attack.

Police have been carrying out raids on workplaces to check migration documents, with $95million added to the police budget this year to bolster the effort.

It comes after failed Uzbek asylum seeker Rakhmat Akilov killed five and seriously injured 14 when he drove a truck into shoppers in the capital back in April.

In 2016, police made about 1,100 unannounced workplace checks, almost three times more than in 2015, and caught 232 illegal immigrants.

Sweden is stepping up efforts to deport illegal migrants after failed Uzbek asylum seeker Rakhmat Akilov killed five people in a truck attack in Stockholm back in April

Prime Minister Stefan Lofven pledged to be tougher on undocumented migrants after the attack, and has pledged an extra $95million for police

A further increase is expected in 2017 as the net widens. Illegal immigrants are also detained through checks at transport hubs, on vehicles or after committing crime.

But despite increased raids authorities often struggle to enforce deportations, with some released because deportation centres are full, because their home country cannot be established, or because their home government refused to take them.

Between January and April police deported just under 600 people, a third fewer than in the same period last year.

Deportations made up a small fraction of the 20,000 rejected asylum seekers who left Sweden last year.

'We have been able to increase the number of people who leave Sweden substantially. But we're listening to the police and we have paved the way for more resources and wider powers,' Johansson said in an interview, adding: 'We will have to increase that number further.'

Expanded police powers include workplace checks without concrete suspicion of a crime, to be allowed from next year, with sharply higher fines for employing illegal immigrants.

Akilov has admitted carrying out the attack, telling police afterward that 'I am a Muslim and I support ISIS'

'We have an unlimited amount of work,' said Jerk Wiberg, who leads the Stockholm police unit in charge of domestic border controls.

A 22-year veteran who has caught thousands of illegal immigrants, Wiberg led a raid at the construction site in May.

That raid netted nine illegal workers - though 40 more escaped by climbing over scaffolding and running along rooftops.

Prime Minister Stefan Lofven issued a degree after Akilov's attack, saying 'no means no' for those whose asylum bids are rejected.

Akilov, whose lawyer said he had admitted to committing the crime, had been in hiding after his asylum request was denied.

The Migration Agency estimated 10,000 asylum-seekers a year will choose to disappear rather than be deported.

Up to 50,000 undocumented immigrants already work in hotels, transport, construction and restaurants, the agency said last year.

Migration Minister Morgan Johansson said a 'dual labour market ... where a growing group lives on the outside of society and remains in Sweden' after having been denied residency was unacceptable.

'It also increases the risk of them being exploited. We cannot have it that way,' he said. 'One way is to go after the employers ... (using) expanded workplace checks.'

There has been an increase in raids on migrants in Stockholm to check immigration papers, but actual deportations in the first four months of the year dropped (file image)

While Sweden has cherished its reputation as migrant-friendly, a recent university study found 51 per cent are in favour of more deportations (pictured, far-right group Nordic Resistance Movement marches in Stockholm to protest against migration)

The crackdown goes against Sweden's long-held self-image as a 'humanitarian superpower' providing a home to migrants fleeing from the Middle East.

But a 2017 study by Gothenburg University showed that attitude may be changing as 52 per cent favoured taking in fewer refugees with 24 per cent opposed.

Two years ago 40 per cent backed reducing refugee numbers with 37 opposed.

The anti-immigration Sweden Democrats are now the second biggest party in polls with support of around a fifth of Swedes.

The Social Democrats, the country's biggest party in every election since 1917 and leader of the governing coalition with the Greens, has been forced to balance its traditional left-wing credentials with the need to enforce immigration laws.

Immigrants themselves have been unnerved. When police burst into a pizzeria in the southern city of Malmo where Ehsanulla Kajfar, a 38-year-old Afghan refugee, was working in May he said he thought they were looking for 'terrorists or drug dealers'.

He was surprised to be handcuffed and placed in the back seat of a police vehicle as tax officials scrutinised the restaurant's employee ledger. He was told his papers were not in order and was taken to a detention centre.

'Sweden used to be a nice country, even when I was living underground,' he said.

'Now although I have a residence permit from Italy and I am registered at the tax agency in Sweden, I'm still locked in a detention centre.'

Nicaraguan Hugo Eduardo Somarriba Quintero, 37, said he was wrongly detained in the big raid in Stockholm in May due to an error by authorities and then released. Migration Agency records confirmed the details of his case.

'But I've lost my job - the company where I was working was dropped from the construction site (because of irregularities in not checking work papers properly). Now I am looking for work and there is no job for me,' he tearfully told Reuters, adding:

'Before there was a lot of tolerance for migrants. Now the laws are harder.'

Muhammad, a 22-year old Afghan who declined to give his family name, has been in hiding for three years in Malmo since his asylum application was rejected.

He has moved three times this year and never stays in a place longer than three months. All his belongings are packed in a suitcase and two plastic bags if he needs to leave in a hurry.

Muhammad relies on food stamps from the church and leftover food from restaurants and grocery stores.

He has learned to avoid the city centre when there is an increase in policing and gets help from other immigrants and volunteers who work for asylum-seekers' rights. They warn each other of police checks and raids through text messages.

'Last time the police made a push to find immigrants, my friend stayed inside for 15 to 20 days,' Muhammad said.

'But I can't stay inside all the time, its too depressing.'