WASHINGTON - It will be difficult for future US administrations to undo President Barack Obama's policy of easing trade and travel restrictions with Cuba because of the benefits associated with the measures, a senior US official said on Friday. 'We've increased the space for this type of travel, people to people exchange, commercial opportunities in ways that are already having a positive impact on the lives of Americans and Cubans,' the official said. 'Turning back the clock on that policy would only take away those opportunities.'