Between 500 and 600 US troops will remain in Syria to keep pressure on ISIS, the chairman of America's Joint Chiefs of Staff has said. President Donald Trump abruptly announced the withdrawal of all 1,000 American troops from the country last month, with the expectation that they would move into Iraq to continue the fight against Islamic State. However, he has since approved a mission to secure oil fields across eastern Syria, requiring a number of troops to stay in place. “There will be less than a thousand, for sure,” Gen. Mark Milley said. "Probably in the 500ish frame, maybe six. It's in that area.""There are still ISIS fighters in the region," he added, explaining: "Unless pressure is maintained – unless attention is maintained on that group – there's a very real possibility there could be a reemergence of ISIS. We committed to [prevent] that. The footprint will be small, but the objective will remain the same."Following the successful targeting by US Special Forces of ISIS leader Abu Bakr al-Baghdadi last month, Washington will be looking for an opportunity to remove his successor, Milley confirmed. "They've apparently replaced him with another leader," he said. "We have a considerable amount of information on that individual. We'll see in the days ahead, weeks ahead, months ahead if he's able to piece together his organization. We'll pay attention to him and, where opportunities arise, we'll go after him as well."But he added that, "it's not just Islamic State – it's other groups. I think we will be [there] for a significant amount of time. It's our national interests to be there to help out."Commenting on the wider region, Milley confirmed that American forces were likely to remain in Afghanistan for the foreseeable future. "We went there to make sure Afghanistan would never again be a safe haven for terrorists that would attack the United States," he said. "That mission is not complete. In order for that mission to be successful, the government of Afghanistan and the Afghan security forces are going to have to sustain their own security forces. That effort is ongoing; it's been ongoing for 18 years. I suspect it will be ongoing for several more years."Turning to Iran, he made it clear that America is choosing not to use military force against the Islamic republic for the time being, but that the picture may change if diplomatic efforts prove ineffective. Tehran has recently stepped up it's nuclear program, and it recently downed a US drone."We hope diplomatic efforts will resolve the nuclear issue with Iran, and we place our faith in the diplomatic efforts," Milley said. "At the same time, we'll make sure that we maintain appropriate levels of military capabilities in the region to defend American interests if required."Commenting on the drone strike specifically, he added: "There's been consequences. Our government has chosen not to react militarily at this time. We have capability. It depends on the scope, scale and nature of any provocation Iran does or any threat they do against US forces in particular, or against US interests or our friends and allies in the region."