(Adds details on previous attack, background on groups) TUNIS, July 2 (Reuters) - Four Tunisian soldiers were killed in a landmine blast during an operation against Islamist militants in the **rth of the country, TAP state news agency reported on Wednesday. Tunisian troops are battling militants from the hardline Islamist group Ansar al-Sharia and other al Qaeda-linked fighters, especially since April when they began a new offensive on militant hideouts in the Chaambi mountains bordering Algeria. Since its 2011 uprising that ousted autocrat Zine El-Abidine Ben Ali and opened democracy, Tunisia has struggled with radical Islamist movements who promote a hardline interpretation of Islam in one of the Arab world's most secular countries. After the killing of two opposition leaders by Islamist militants last year, the small **rth African state declared Ansar al-Sharia a terrorist group.