KABUL (Reuters) - Taliban militants ambushed a government checkpoint in Balkh province in Afghanistan’s north, killing at least 15 members of the security forces, local officials said on Tuesday, the latest in an escalating series of insurgent attacks.

More than 18 years after U.S.-led forces toppled the hardline Taliban regime, Afghan forces are suffering record casualties with Afghan and U.S. officials warning that the toll is not sustainable.

Despite a winter lull in fighting due to heavy snowfall in the mountains where the Islamist Taliban typically rest and regroup ahead of their annual spring offensive, assaults on vulnerable security checkpoints continue across the country.

Mohammad Yousuf, the district governor of Dawlatabad where the incident took place said, said dozens of Taliban insurgents targeted a checkpoint manned jointly by Afghan soldiers and intelligence agents, killing 15.

An Afghan Defence Ministry statement said seven soldiers had been killed. The ministry tends not to provide exact figures on its casualties sustained in fighting with the Taliban.

The Taliban, fighting to oust foreign forces and overthrow the Western-backed Kabul government, claimed responsibility and said at least 20 members of the security forces were killed, six wounded and four captured. The Taliban often inflates casualty figure they inflict on Afghan government forces.

As Afghan security force casualties mounted this year, the government pulled back from hundreds of checkpoints in isolated areas that acted as a magnet for Taliban attacks.

Tens of thousands of Afghan civilians, members of the security forces and 2,400 U.S. military personnel have been killed in fighting since 2001.