KANDAHAR, Afghanistan - A suicide bomber detonated his explosives near a police checkpoint and a bank Saturday in southern Afghanistan, one of two attacks in the heartland of the insurgency that killed 18 people. Separately, a NATO service member was killed by insurgents in the country’s east, according to a military statement.

No group immediately claimed responsibility for any of the attacks, but Afghan President Hamid Karzai blamed the bombings on the Taliban. The militants have escalated their activity as U.S.-led foreign forces reduce their presence in the country and are in the final phase of handing over responsibility for security to Afghan troops.

Karzai said the militants should stop taking orders from foreigners - a veiled reference to Pakistan, whose intelligence services are alleged to be in league with the Afghan Taliban. The president said the security transition is nearly complete and the militants were desperate to derail it.

“Taliban leaders and commanders must understand that with such crimes theywill achieve nothing, but only be hated and disgust the people and God,” Karzai said in a statement. “They must be accountable for the Muslim nation of Afghanistan.”

Javed Faisal, a spokesman for the provincial governor, initially said the suicide bomber was in a car that was being searched by police, but later said new information indicated the bomber was on foot. The bank branch, several small shops and vehicles were damaged.

Faisal said at least six people died - four civilians, one police officer and one private security guard. Another 24 people were wounded, most of them civilians.

Taliban spokesmen did not immediately respond to requests for comment.

Another 12 people were killed in an ambush involving a roadside bomb in Sangin district in Helmand province, also in the south, on Friday evening, said Omer Zwak, a spokesman for the provincial governor. Sangin is the scene of an ongoing operation by Afghan forces against the Taliban.

Zwak said 11 men and one woman died in the attack. Such attacks typically target security forces, but, in this case, “the victims are all civilians and had no link with the government,” Zwak said.

On Saturday evening, a handful of insurgents attacked a police compound in Qalat, a city in southern Zabul province. The insurgents then holed up in a nearby house, under siege from security forces, said Mohammad Jan Rasoolyar, the province’s deputy governor.

Rasoolyar said one police officer and one militant were killed in the fighting, while three police officers were wounded. But as of Saturday night, the standoff had not ended, and Rasoolyar said the situation presented a challenge for Afghan security forces because women and children were also in the home taken over by the militants.

The official described the militants as Taliban fighters and said there were at least six.

The NATO service member died in “a direct fire attack by enemy forces in eastern Afghanistan” on Saturday, according to a statement from the military alliance. The statement did not give any further details.

Information for this article was contributed by Rahim Faiez of The Associated Press.

Front Section, Pages 9 on 09/01/2013