The site of a violent attack is blocked by the police in Stockholm, Sweden, April 8, 2017. (Xinhua/Shi Tiansheng)

STOCKHOLM, April 9 (Xinhua) -- Swedish Prime Minister Stefan Lofven on Sunday vowed to enforce the country's deportations effectively in wake of the Stockholm truck attack, of which the main suspect the police said was a 39-year-old man denied of residence application in Sweden in 2016.

According to the Swedish TT news agency on Sunday, the 39-year-old suspect applied for a residence permit in Sweden in 2014. In June 2016, Swedish Migration Board rejected his application and in December he was given a deadline of four weeks to leave the country.

However, in February 2017, the police was unable to enforce deportation order as the man was not at the address he registered.

Lofven said this made him frustrated and that one must be deported if being rejected of application for a residence permit or an application for asylum.

"We need to improve the effect," Lofven told TT, adding that "it is also important for confidence in the system."

The 39-year-old man from Uzbekistan is current under police custody and is believed as the driver of the deadly truck which rammed into crowd, killing four people and injuring 15 others in central Stockholm on Friday.

Swedish paper Aftonbladet reported earlier that the Uzbek man had expressed support for the Islamic State (IS) online and was known to intelligence agencies.