A Taliban suicide bomber killed at least seven people and wounded 42 others in southern Afghanistan on Wednesday, an official said, in an attack targeting police.

"A suicide bomber detonated an explosive-filled car in a car park near the main police headquarters in Lashkar Gah," Omar Zhwak, a spokesman for the governor of Helmand province, told reporters.

In addition to four soldiers, a girl and two women were among those killed, Zwak said, expressing fears that the death toll could rise further.

Children were studying at a nearby mosque when the explosion took place, Zhwak said, adding they were injured in the violence.

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Authorities said that a group of police officers who had lined up to collect their pay were the target, according to Al Jazeera's Jennifer Glasse, reporting from Kabul.

"As there have been so many threats on Afghan banks that pay out government and security forces, there is actually a bank branch inside the police station," she said. "Police officers were lined up when the attack happened."

Ismail Jan, whose shop was damaged by the blast, said the explosion happened when a white car rammed into a convoy of six armoured military vehicles as they passed through a checkpoint inside the car park.

"I was thrown [by the blast], and when I got up I saw women and children covered in blood," Jan told the AFP news agency. "I also saw a number of wounded children taken out of a nearby mosque."

The Taliban effectively controls or contests 10 of 14 districts in Helmand.

Intensified fighting

Intensified fighting last year forced thousands of people to flee to Lashkar Gah from neighbouring districts.

Early in July, an American soldier was killed, and two others were wounded in an attack in Helmand province while conducting operations against the Taliban.

Since it launched an annual spring offensive in late April, the Taliban has been mounting lethal assaults on the Afghan army and police outposts in Helmand.

The attack comes a day after US President Donald Trump announced that the US would continue fighting in Afghanistan to avoid the "predictable and unacceptable" results of a rapid withdrawal from the country.

In response, the Taliban said that Afghanistan would become a "graveyard" for American soldiers.

Trump did not provide the number of additional troops that would be sent to the conflict, the US' longest war, although US officials said before the speech they expected him to go along with a Pentagon recommendation for nearly 4,000 new troops.