Vatican City - Pope Francis on Wednesday insisted that indigenous peoples had to give prior consent for any economic activity on their ancestral lands - an indirect critique as the Donald Trump administration seeks to advance construction on a $3.8 billion oil pipeline over opposition from American Indians.The pope met representatives of indigenous peoples attending a UN agricultural meeting and said the key issue facing them is how to reconcile the right to development with protection of their cultures and territories."In this regard, the right to prior and informed consent should always prevail," he said. "Only then is it possible to guarantee peaceful co-operation between governing authorities and indigenous peoples, overcoming confrontation and conflict."The Cheyenne River and Standing Rock Sioux tribes are suing to stop construction on the final stretch of the Dakota Access pipeline, which would bring oil from North Dakota's rich Bakken fields across four states to a shipping point in Illinois.The tribes say the pipeline, being built by Texas-based Energy Transfer Partners, threatens their drinking water, cultural sites and ability to practice their religion, which depends on pure water. The last piece of the pipeline is to pass under a reservoir on the Missouri River, which marks the eastern border of both tribes' reservations.The company has insisted the water supply will be safe.The pipeline dispute has led to protests and clashes in recent months that have resulted in some 700 arrests.