By Renee Maltezou and Lefteris Papadimas ATHENS (Reuters) -Iran has agreed to release the crews of two Greek tankers it seized in May in response to the confiscation of oil by the United States from an Iranian-flagged tanker in Greece, a Greek seafarers union said. The months-long diplomatic impasse has strained relations between Athens and Tehran as tensions grow between Iran and the United States. The crews will be replaced, allowing their return to their countries of origin soon, the PENEN union for seafarers on commercial ships said on its website on Sunday. It was not clear whether the tw…