Malaysia has stepped up security at its borders in case Malaysian militant fighters try to return home after Iraqi forces launched a major offensive to take back the Islamic State stronghold of Mosul, deputy prime minister Ahmad Zahid Hamidi said on Tuesday. Iraqi government forces launched a US-backed offensive on Monday to drive Islamic State from the northern city of Mosul, the group’s last major stronghold in Iraq. Around 4,000 to 8,000 militants, a mix of Iraqi and foreign fighters, are estimated to be in the city. Ahmad Zahid told a news conference that Malaysian airport and border security had been increased, while illegal routes commonly used by smugglers were being monitored. Why are Malaysia, Singapore nervous as Iraq looks to retake Mosul from Islamic State? “We have been exchanging intel with international intelligence agencies, and we have a suspect list which includes names of those we believe have ties with Daesh,” he said, using an alternate name for Islamic State. Ahmad Zahid did not state how many Malaysians were currently in Mosul but police figures released last month showed that 90 Malaysians had joined Islamic State in Syria and Iraq since 2013. In August, Malaysia revoked the passports of 68 Malaysians who had been identified as leaving the country to join Islamic State. Returning fighters would be detained and sent for deradicalisation, Ahmad Zahid said. A total of 137 people have been arrested for either planning to join Islamic State overseas, returning to Malaysia after joining the group, or sending funds to the group, he added. Retaking Iraq’s Mosul, an Islamic State stronghold, will be a complex offensive Defence Minister Hishammuddin Hussein had said on Monday intelligence sources suggested that thousands of Islamic State members would make their way back to their countries of origin, or find safe havens in regions such as Southeast Asia, if the Mosul offensive succeeds. “We have to be very proactive,” he was quoted as saying by state news agency Bernama. Authorities in Muslim-majority Malaysia have been on high alert since Islamic State-linked militants carried out an armed attack in the capital of neighbouring Indonesia in January. In June, eight people were injured when two Islamic State supporters threw a grenade into a bar on the outskirts of Kuala Lumpur, the first successful attack by the group in the Malaysia.