The Taliban commander overseeing an assault on the key southern city of Lashkargah is one of 5,000 former prisoners released by the Afghan government last year under pressure from the U.S., Afghan and Western officials say.

The commander, Mawlavi Talib, is one of thousands of former prisoners freed to further peace talks who have returned to the battlefield to join the Taliban onslaught against cities around the country, Afghan officials say. The presence of the Taliban fighters is a fresh strain in relations between Washington and Kabul, as the U.S.-backed government struggles to fend off Taliban attacks throughout the country.

Afghan officials said fierce fighting continued Tuesday in Lashkargah, the provincial capital of Helmand province, after heavy U.S. airstrikes and a counterattack by U.S.-trained Afghan commandos overnight. Under Mr. Talib’s command, hundreds of Taliban have pushed toward the city center over the past week. On Tuesday they launched an attack on Lashkargah’s prison in a bid to release more inmates and recruit them into the fight.