WASHINGTON, June 17 - President Barack Obama will announce efforts to protect areas of the Pacific Ocean controlled by the United States from overfishing and environmental damage, the White House said on Tuesday. The measure is likely to please environmental advocates but annoy political opponents, who accuse Obama of conducting an imperial presidency by frequently resorting to unilateral actions without seeking congressional approval. Obama's proposal, due to go into effect later this year, would create a vast marine sanctuary and is part of an effort to safeguard more ocean territory, the White House said. It was first reported by the Washington Post. "We've already shown that when we work together, we can protect our oceans for future generations. So let's redouble our efforts," Obama said in a statement. The president's plan would expand the Pacific Remote Islands Marine National Monument to nearly 782,000 square miles (2.03 million sq km) from 87,000 square miles (225,000 sq km) and would affect seven islands and atolls controlled by the United States, the Post reported. A comment period over the summer will allow the Commerce and Interior departments to hear objections and fine-tune the plan.