Police in North Dakota began clearing a group of Native American and environmental protesters from an encampment near an oil pipeline construction site on Thursday in a move that could escalate tensions in a standoff that has lasted several months.br "Protesters' escalated unlawful behavior this weekend by setting up illegal roadblocks, trespassing onto private property and establishing an encampment has forced law enforcement to respond at this time," Morton County Sheriff Kyle Kirchmeier, said in the statement.