Nato and Afghan troops were pouring into the south of Afghanistan after reports from local officials that a strategically vital district next to the country’s second largest city was being overrun by Taliban fighters.

Western officials said 700 Afghan soldiers have been flown from the capital to take part in what is expected to be a major operation to dislodge militants, who appear to be following up on the spectacular jailbreak on Saturday when militants freed around 400 of their compatriots.