WASHINGTON — The United States began accelerating and intensifying its campaign against the Islamic State in January, at President Trump’s direction, but it was not putting civilians in war zones at greater risk, the defense secretary said Friday.

Defense Secretary Jim Mattis said Mr. Trump’s decision to delegate more authority to commanders had put greater pressure on militants in Syria, Yemen and Somalia. And he said a tactical shift to surround Islamic State strongholds in Mosul, Iraq, and Raqqa, Syria, was devised not only to “annihilate” thousands of fighters hunkered down there, but also to prevent them from returning to their home nations in Europe, Africa and the Middle East, possibly to commit mayhem in those places.

In an hourlong news conference at the Pentagon, Mr. Mattis took issue with human rights groups and other critics who have warned that the military’s new authorities freeing them from Obama-era constraints on airstrikes and raids against insurgents in Yemen and Somalia have jeopardized the safety of civilians in those countries.

“I want to emphasize here there has been no change to our rules of engagement, and there has been no change to our continued extraordinary efforts to avoid innocent civilian casualties,” Mr. Mattis told reporters.