In the bloody aftermath of two deadly suicide bombings near Kabul airport on Thursday, top US military veterans are warning of "nightmare" scenarios if US forces delay their exit from Afghanistan beyond August 31."If we stay against the will of the Taliban -- on Wednesday morning -- the nightmare scenario is light machine gun fire, and a dozen mortar rounds, closing down their evacuation of our forces. At which point, we've got a real problem," retired four star general Barry Richard McCaffrey said in a television interview on Thursday.Earlier this week, Taliban made it clear that they would not accept US military presence at the airport beyond August 31. That date is a result of deals struck with the Taliban -- first by the Donald Trump administration and follow-through action by the Joe Biden administration.Trump committed to the Taliban that the US would get all American troops and contractors out by May 2021. Biden announced in April that he would have all forces out by September. The US is now 12 days into its chaotic evacuation from Afghanistan and five days before its scheduled end.The Pentagon confirmed that at least 13 US service members have been killed in the Thursday attacks and at least 18 are reported wounded.ISIS-K, far more radical than the Taliban who seized power less than two weeks ago, claimed responsibility for the Kabul bombings.As many as 1,000 Americans and millions of Afghans are still struggling to get out of Kabul. Biden has vowed that US military commanders in Afghanistan will complete the evacuation mission. "And we will," he said. "We will not be deterred by terrorists.""Keep your eye on the challenge here," McCaffrey told NBC. "The challenge is to complete the extraction of US forces, and then deal with the consequences."