ISTANBUL (Reuters/AFP) - The top US and Nato commander in Afghanistan said on Thursday that the security situation remains serious but is no longer deteriorating, offering a more upbeat assessment than other military and intelligence officials. General Stanley McChrystal and other top commanders made dire warnings last summer about Afghanistans worsening security situation, prompting US President Barack Obama to overhaul the war strategy and to send 30,000 more troops to combat a resurgent Taliban. I still will tell you that I believe the situation in Afghanistan is serious. I do not say now that I think its deteriorating. I think and I said that last summer, and I believed that that was correct. I feel differently now, McChrystal told reporters in Istanbul on the sidelines of a meeting of Nato defence ministers. However, McCh-rystal also cautioned: Im not prepared to say that weve turned the corner. The US urged Nato allies to send thousands of trainers to help strengthen Afghan security forces and exploit a window of opportunity to defeat the Taliban and Al Qaeda. It is important that our allies contribute their forces to this cause in as timely a manner as possible just as we are, Pentagon spokesman Geoff Morrell told reporters in Istanbul. We need to seize that window of opportunity... and get the Afghan national security forces to the position they need to be to transition into a leadership role on the security side, he said. A senior US official said up to 1,700 instructors were needed for the police and army, while up to 2,500 additional mentors were required to work alongside the national security forces.