File photo of passengers, among them migrants and refugees, exiting the German ferry terminal in Goteborg Thomson Reuters Under huge strain from an influx of unaccompanied children seeking asylum, the Swedish government faces political pressure to undertake medical tests like X-rays to vet the age of young refugees despite opposition from doctors and lawyers.

The controversy reflects tensions over surging immigration into the Nordic country of 10 million after a public backlash that saw controls reimposed on the border with Denmark, from which most migrants have entered Sweden.

Sweden took in 163,000 asylum seekers last year, the most per capita in Europe. They were among more than one million who streamed into the continent, fleeing increasing conflict and deprivation in the Middle East, Africa and Asia.

More than a fifth, 35,000, of those reaching Sweden have been unaccompanied children, stretching services like schools.

Reports of violence and assaults at centers for minors have added to the public disquiet and hardened anti-immigrant sentiment in a country long renowned for its humanitarian open-door policy toward the wretched of the earth.

Many have arrived without IDs, leaving Sweden the task of checking their real ages to ensure adults - defined as age 18 or over - were not pretending to be minors to secure asylum.

Migrants walk through thick mud as they cross the Macedonia / Serbia border in the southern Serbian village of Miratovac on January 27, 2016 in Miratovac, Serbia. Getty Images

Worried young adults may be swelling these ranks, many of Sweden's opposition parties are calling for medical tests. Even the government has called for more non-medical testing while it awaits reform proposals in April to break the deadlock.

There is a great incentive to claim to be a minor. Applicants have greater access to housing and schools and less chance of being deported.

"At the moment, very few, if any, age assessments are being done in Sweden," Fredrik Beijer, Director of Legal Affairs of Sweden's Migration Agency, told Reuters.

Efforts to confirm ages have been hampered by the inability of authorities to carry out medical tests - such as X-rays of teeth and hands. The government said in November it wanted medical tests. But while such tests are not illegal, doctors have refused, saying they are inaccurate.

"We believe that for a decision that has such large consequences in an individual's life, one must require higher precision," said Anders Hjern, a spokesman for the Swedish Paediatric Society.

Children Nor, Saleh and Hajaj Fatema from Syria sleep outside the Swedish Migration Board in Marsta, outside Stockholm, Sweden. Picture taken January 8, 2016. Reuters

But doubts have not stopped the center right Moderates, Sweden's biggest opposition party, calling on the migration agency to hire doctors for medical checks in an effort to lower the number of children arriving without families.

"Unaccompanied minors make up around 20 percent of asylum seekers but they cost about half the migration budget," said Hanif Bali, an Iranian-born lawmaker for the Moderates. Bali himself came to Sweden as an unaccompanied minor at age three.

"Out of my own experience, because I have lived in these kinds of homes, the environment becomes much harsher when you have older people there. You get prison rules and many children get caught up in the older people's trouble-making."

Many other European Union countries do carry out medical tests. Austria, for example, allows doctors to do "age determination reports" that include checks on teeth as well as genital development. In Italy, medical-age assessments, such as X-rays, can be carried out by court order.

Sweden shocked by assaults

Sweden's Minister for Foreign Affairs Margot Wallstrom gives a statement to media regarding the attacks in Paris, in Stockholm, Sweden Thomson Reuters

The issue of refugee minors is especially sensitive in Sweden. Reports of assaults in overcrowded minors' centers - including a 22-year-old female Swedish employee of one center who was stabbed to death last week - have contributed to a sense authorities are overwhelmed.

"The risk of disputes and discontent is obvious, and some small detail may trigger conflict," said Thomas Svensson, head of social affairs for the Emmaboda municipality in southeastern Sweden, where staff at a home for unaccompanied minors locked themselves in a room as 19 migrant youths rioted.

The influx of minors also carries big fiscal costs. Sweden last year had to find an extra 70,000 school places due to asylum seekers, on top of the 100,000 pupils that normally enter the school system for the first time in any given year.

Half of unaccompanied minors have been registered as between 16 and 17, often making age confirmation difficult and sparking accusations from the likes of the far-right Sweden Democrats - the third biggest party in parliament - that adults are taking advantage of soft controls to enter the country.

Even without medical tests, some 667 minors had their age "adjusted" between January and November last year, according to the justice ministry. The data does not show if it was adjusted to above 18. That compares to 363 cases for all 2014.

Thousands of people attend a "Refugee Welcome" demonstration at the Gotaplatsen square in Gothenburg, Sweden, Wednesday Sept. 9, 2015. AP/Adam Ihse

Proud of Sweden's decades-old tradition as a self-proclaimed "humanitarian superpower", the government regards most refugee minors as bona fide refugees fleeing war.

Immigration supporters say Swedes have been unduly influenced by a media frenzy linking migrants with crime that has little to do with reality. For example, despite reports of refugees being associated with sexual assaults, reported rapes fell 12 percent last year in Sweden. Thefts were down 2 percent.

The debate is part of a crisis that has cut center-left Prime Minister Stefan Lofven's support to record lows in polls due to a popular sense that his government is largely helpless to stop a migrant influx seen as threatening Sweden's generous welfare state and vaunted social stability.

In a sign of mounting political frustration, Migration Minister Morgan Johansson called on Sunday for the migration agency to carry out more non-medical tests, such as interviews with children.

(Additional reporting by Johan Ahlander and Johan Sennero in Stockholm, Shadia Nasralla in Vienna and Steve Scherer in Rome; Editing by Mark Heinrich)