Afghan President Hamid Karzai warned troops to avoid civilian casualties during a major assault against the Taliban in the southern Afghan province of Helmand that began Saturday. Thousands of Afghan and Western troops, led by US Marines, launched Operation Mushtarak, or "together," against Taliban militants in the town of Marjah, said to be the militants' last bastion in the south of the country. In a statement, Karzai "instructed all Afghan and international troops to exercise absolute caution to avoid harming civilians." "The president urges Afghan and foreign military authorities to continue to consult the local people, fully coordinate their actions and to avoid using air power in areas where civilians can be at risk." The Western-backed leader also called on the rebels -- many of whom are opportunistic rather than ideologues -- to lay down their arms and rejoin civilian life. "President Karzai calls upon all Afghan Taliban to use this as an opportunity to renounce violence and re-integrate into civilian life alongside other Afghans for the welfare of their country," the statement added. Five insurgents were killed in the first contacts with the troops dropped by more than 60 helicopters over the small town, Sher Mohammad Zazai, an Afghan army general told reporters. The Taliban, whose 1996-2001 regime was overthrown in a US-led invasion, are the main militant group behind an increasingly deadly insurgency aimed at toppling Karzai's government. Operation Mushtarak is the biggest assault against the rebels since Washington began sending more troops to the strife-torn country to defeat the Taliban and help Kabul expand its influence across the country. US President Barack Obama announced in December a surge of an extra 30,000 troops, supplemented by another 10,000 from NATO, arriving through to August and mostly deployed to the troubled southern provinces. There are 113,000 foreign mainly NATO troops based in the country.