BAGHDAD, Aug 20 (Reuters) Iraqi security forces launched today an offensive to take back the city of Tal Afar, their next objective in the US-backed campaign to defeat Islamic State militants, Prime Minister Haider al-Abadi said. "You either surrender, or die," Abadi said in a televised speech announcing the offensive, addressing the militants. A longtime stronghold of hardline Sunni insurgents, Tal Afar, 50 miles (80 km) west of Mosul, was cut off from the rest of the Islamic State-held territory in June. The city is surrounded by Iraqi government troops and Shi'ite volunteers in the south, and Kurdish Peshmerga fighters in the north. About 2,000 battle-hardened militants remain in the city, according to US and Iraqi military commanders. Islamic State's self-proclaimed "caliphate" effectively collapsed last month, when US-backed Iraqi forces completed the takeover of the militants' capital in Iraq, Mosul, after a nine-month campaign. REUTERS PS 0637