Sweden will not be cowed by "these heinous murderers," Swedish Prime Minister Stefan Lofven said on Friday after a truck attack in Stockholm killed four.

"Today we have been subjected to a terrible attack at the heart of our capital. We know that four people are dead, more are injured and a whole country are gathered in grief, anger and determination," media Lofven as saying.

According to Lofven, the purpose of the attack is to spread fear and sow discord. But he said, "Our message will always be clear: You can not subjugate us, you can not control our lives, you can never win."

"We have also decided tonight to strengthen border controls," Lofven announced.

Meanwhile, Swedish Security Service SAPO identified the truck attack as a terrorist attack saying it handled the case as a terrorist crime.

"Our main objective is to ensure that there won't be more attacks," said Johan Olsson, chief operating officer of the Security Service.

Swedish Television reported that border controls would be introduced this evening. It is unclear how long they would be in force. 

But SAPO said earlier it would not raise the threat level in wake of the suspected terror attack, Swedish Television reported on Friday.

"It is continuously reviewed. For a long time it has been on a three on a five-point scale, and the threat level remains at a third. Within the level of fit that an attack may be happening," SVT quoted SAPO press secretary Karl Melin as saying.

"We regard this as a special case and we are working closely with the police to act quickly. It's about the intelligence to find out who or what is behind," said Melin.

Death toll of Stockholm attack rose to four, after one victim died in hospital.

Swedish Television quoted hospital sources at Karolinska Institutet as saying that the number of the injured from the attack had risen to 15.