Burning NATO supply trucks are seen in Samangan, north of Kabul, Afghanistan, on July 18, 2012. Afghan officials say a magnetic bomb placed on a truck exploded and destroyed 22 NATO supply vehicles. (Javed Basharat/AP)

Assailants claiming to belong to the Taliban destroyed 22 trucks carrying supplies for NATO forces in a normally secure region of northern Afghanistan on Wednesday, while insurgent attacks on two army checkpoints in the south and east left 12 Afghan soldiers dead.

All three incidents underscore the tenuous security situation as NATO continues its summer troop drawdown — an early stage of the phased withdrawal that will continue through 2014. Some worry that the drawdown could put too much stress on Afghan troops in dangerous districts and provinces.

Nine soldiers were killed in a predawn attack on an Afghan army checkpoint in restive Helmand province, Afghan officials said. The bombing of supply trucks occurred at about the same time in the northern province of Samangan, an area that serves as a key route for the delivery of materials to NATO and U.S. troops.

Afghan authorities were investigating whether the checkpoint attack in Helmand’s remote and arid Washer district was organized with the help of an Afghan soldier whose whereabouts since the raid were unknown, said Daud Ahmadi, a spokesman for Helmand’s governor.

“Police reinforcements were sent, and they killed seven Taliban in gun battles,” Ahmadi said.

“We suspect the missing soldier was involved in a plot in the killings of the nine army soldiers but have to investigate this point,” he said.

Ahmadi did not have further details about the attack, the deadliest single strike against the Afghan army in months.

Also Wednesday, a man wearing an Afghan army uniform blew himself up at a checkpoint in the eastern province of Logar, killing three Afghan troops, the Associated Press reported. The Taliban asserted responsibility for the attack.

In Samangan, the Afghan-owned supply trucks, including 18 fuel tankers, were parked when they were struck by a bomb that the Taliban later said it had planted, according to the AP. One person was reported wounded in the blast.

The movement of NATO supplies has been a sensitive issue for much of the past year, as the United States struggled to persuade Pakistani officials to allow its trucks to cross the border. The transportation of military supplies has been further complicated recently by a string of attacks on such convoys, which are protected by private Afghan security companies.

Dozens of trucks have been attacked along the main highway from Kabul to Kandahar. Although attacks in the north are less common, officials said several similar incidents have occurred in the region in recent months.

Samangan straddles a highway that has been used in recent months as the key overland supply line for foreign forces after Pakistan barred NATO supply convoys from its territory to protest the killing of 24 Pakistani troops in a U.S. air attack on two border outposts in November. Islamabad has lifted the blockade in recent weeks, but foreign troops still rely on the northern Afghan region as a route for supplies from Central Asia.

Samangan was also the scene of a suicide bombing Saturday that killed at least 19 people at a wedding ceremony in a hotel. Among the dead were a number of prominent officials, including the bride’s father, Uzbek member of parliament Ahmad Khan Samangani, a former warlord.

Until the suicide attack and the destruction of the trucks, Samangan had been among the few relatively secure parts of Afghanistan. The Taliban maintains fewer pockets of resistance in the north compared to the south and east, the traditional power bases of the Islamist militants.

A cabinet minister and provincial governor escaped unhurt Sunday when the Taliban attacked their motorcade in Baghlan province, adjacent to Samangan.

Kevin Sieff contributed to this report.