WASHINGTON - Despite the humiliating treatment meted out to senior Pakistani military officers at Washington airport, the United States Wednesday expressed "regret" only - not an apology - and gave no assurance that it won't happen in future. "These things are happening and they're just a fact of life and will be as long as air travel remains the primary target of international terrorism," State Department spokesman PJ Crowley told reporters while responding to questions about Monday night's incident at Dulles Airport outside Washington. However in a late development on Thursday night, US Under Secretary of States for Defence, Michelle Flournoy, telephoned her Pakistani counterpart, Lieutenant General (Retd) Syed Ather Ali and apologised over mistreatment meted out to a high-level Pakistani military delegation at the US airport the other day, according to ISPR sources. The Pakistani Defence Secretary protested over the incident and expressed brief concerns in this connection. He also emphasised on the need for devising an institutionalised mechanism to prevent such incidents in future. The US Defence Secretary assured him that an institutionalised mechanism would be formed with mutual consultations. She also expressed her deep condolence over the life losses in recent floods in Pakistan and assured all US' assistance to help Pakistan tackle this calamity. Syed Ather Ali acknowledged the US military assistance in rescue and relief operations being carried out in the flood-hit areas of the country and thanked the US Government for this gesture. However in Washington, while replying to a question Crowley said, "We have been in communication with Pakistan and we have expressed our regret." Crowley also said that efforts would be made to reschedule the talks in Pakistan. Responding to questions about the incident, the Spokesman said, "It is hard to interpret exactly what happened on board the airplane. It was regrettable. We think that there was a misunderstanding, miscommunication between the Pakistani contingent and the aircrew." The nine-member delegation of high-ranking Pakistani officers boarded the United Airlines Flight 727 from Washington to Tampa late Monday but abandoned the flight in protest against the "inappropriate behaviour" of the crew and US security officials. The delegation was due to attend a conference at US Military Central Command Headquarters in Tampa, Florida, on Tuesday. But following interrogations and unwarranted checks, they decided to cancel the trip and return home. "All this started, when one of the officers who - weary of a long day of travel that began in Islamabad - said, "I hope this is my last flight," or words to that effect. "The delegation was subjected to unwarranted security checks at Washington airport by the US Transport Security Agency," the Pakistan Army said in a statement. "However, as a result of these checks, military authorities in Pakistan decided to cancel the visit and called the delegation back," it said. "It is regrettable. Unfortunately, it is the environment that we find ourselves when it comes to air travel these days," Crowley told reporters. "All of us are subject to air travel and it's become a lot less fun than it used to be. But this is something that happens day in and day out to Americans and Pakistanis and the citizens of other countries," he added. "We've already had conversations. We're going to have more conversations with Pakistan regarding this incident," Crowley said. Earlier, the Department of Defence (DoD) denied reports that either Pentagon or the CENTCOM have apologised for the incident. "I can't speak for the entire Department. If some individuals told them that I am sorry, that is different than a DoD apology. But I do not want to rule out that someone in the Department might have talked to them," Pentagon spokesman Dave Lapan told reporters. "That was an unfortunate incident that these gentlemen were coming here to participate in a conference in CENTCOM," Lapan said, adding that he does not have all the facts of the incident and cannot rule anything out.