Iran has released 10 US sailors held for entering its territorial waters, in a swift resolution of an incident that tested newly-improved US-Iranian ties. They were detained Tuesday after one of their two vessels broke down during a training mission in the Gulf. Iranian state media said the group was released into international waters after apologising. It comes at a sensitive time, as the US and Iran try to implement the deal on Iran's nuclear activities. The incursion was "unintentional", a statement from the Revolutionary Guards quoted by state media said. Earlier, the naval commander of the Guards, General Ali Fadavi, said investigations found a navigational failure was to blame. "We have concluded that passage of Americans in our territorial waters was not a hostile passage or for espionage or similar acts," he told Iranian television. A Pentagon statement said the Navy sailors had been safely returned, and that an investigation was under way as to how they entered Iranian waters. "Around the world, the US Navy routinely provides assistance to foreign sailors in distress, and we appreciate the timely way in which this situation was resolved," said Defence Secretary Ash Carter, according to BBC.