The Syria strike — on Tuesday near the town of Tabqah, which Syrian fighters and American advisers are trying to capture — was the third American-led airstrike in a month that may have killed civilians or allies. Earlier bombing runs killed or wounded scores of civilians in a mosque complex in Syria and in a building in the west of Mosul, Iraq.
“We have the greatest military in the world,” Mr. Trump said. “We have given them total authorization, and that’s what they’re doing, and frankly, that’s why they’ve been so successful lately.”
American commanders in Iraq and Syria have been given more authority to call in strikes, a loosening of the reins that began in the last month of the Obama administration. But some national security experts said that Mr. Trump and the Pentagon risked inflaming anti-American sentiment in the Muslim world with their approach to fighting the Islamic State.
The number of civilian casualties reported in American-led strikes in Iraq and Syria has increased since Mr. Trump took office, and March was the deadliest month for civilians ever recorded by Airwars, a group that tracks bombings. Reports of civilian casualties in Iraq and Syria jumped to 3,471 from 1,782 the month before, the group said.
American officials have attributed the rising number of strikes and the increased danger to civilians to the fact that the fight is moving to the densely populated urban battlefields of Mosul and Raqqa, the Islamic State’s self-proclaimed capital in Syria. They say they try to avoid civilian casualties, while the militants deliberately kill anyone who stands in their way.
In addition to the greater leeway granted to commanders in Iraq and Syria, Mr. Trump has relaxed some of the rules for preventing civilian casualties when the military carries out counterterrorism strikes in Somalia and Yemen.
“Trump has ceded responsibilities to his military commanders, and it appears he’s paying little attention to operational details,” said Derek Chollet, who was the assistant secretary of defense for international affairs in the Obama administration.
“Here’s the question,” Mr. Chollet added. “Trump takes great pride in his authorizing the military when things go well, but one wonders if he’ll have the same sense of shared accountability when things go wrong, as they inevitably do.”
Thursday’s strike in Afghanistan — using a 20,000-pound bomb that cost $16 million, and more than $300 million to develop — hit a tunnel complex in the Achin district of Nangarhar Province, according to a statement from the United States military in Afghanistan. The statement did not say how many militants were killed, or whether the bombing caused any civilian casualties.
The weapon is so big that, while the cargo plane is in the air, the bomb rolls out of the rear on a pallet, pulled by a drogue parachute. It is designed to destroy tunnels and other underground facilities, and its blast radius is estimated to stretch a mile in every direction.
The strike against tunnels and caves reflects the ever-changing nature of the war in Afghanistan, now in its 15th year.
During the years of intense fighting in Afghanistan, the United States dropped a handful of similar bombs to destroy caves believed to be used by the Taliban and Al Qaeda, as well as to frighten troops dug into trenches who were not immediately killed. The military offered a similar rationale on Thursday for using the bomb — a successor to the “daisy cutter,” a heavy bomb designed for the instant clearing of large sections of jungle in Vietnam.
Islamic State fighters in Afghanistan “are using I.E.D.s, bunkers and tunnels to thicken their defense,” said Gen. John W. Nicholson Jr., the United States commander there, referring to improvised explosive devices. “This is the right munition to reduce these obstacles and maintain the momentum of our offensive.”
Afghan security officials said their ground forces had advanced on the Tangi Asadkhel area of Achin but met firm resistance from Islamic State militants who were launching attacks from six mountainside tunnels. The Afghan forces retreated and asked for airstrikes.
“The ground forces could not do it, so the Americans bombed the area,” said Gen. Dawlat Waziri, a spokesman for the Afghan Ministry of Defense.
The Afghan government was kept “in the loop” about the bombing, said Shahhussain Murtazawi, a spokesman for the president.
While the damage from the bombing, which occurred at night in a remote area, was unclear, the strike quickly brought backlash. Hamid Karzai, Afghanistan’s former president, was among those who condemned it.
“This is not the war on terror but the inhuman and most brutal misuse of our country as testing ground for new and dangerous weapons,” Mr. Karzai wrote on Twitter. “It is upon us, Afghans, to stop the USA.”
Jon B. Alterman, director of the Middle East Program at the Center for Strategic and International Studies, said the Pentagon was being given leeway to carry out strategy without being told what, exactly, the overarching strategy is. “What they haven’t been given is a lot of strategic guidance to work with,” he said. “They can affect things, but without a guiding strategy, it’s hard to be sure you’re having the desired effect.”
Tuesday’s strike in Syria was requested by coalition allies on the ground near Tabqah, according to the United States Central Command, which oversees combat operations in the Middle East. The fighters had “identified the target location as an ISIS fighting position,” it said in a statement, using another name for the Islamic State.
The military said the target location turned out to be a “fighting position” for the Syrian Democratic Forces, who have been fighting the Islamic State alongside the United States.
“The coalition’s deepest condolences go out to the members of the S.D.F. and their families,” Central Command said in the statement, calling the episode “tragic.” Military officials said the cause was being investigated.
But the increased casualties in Syria “cannot be explained away simply by the increased tempo of the war,” said Chris Woods, director of Airwars.
He noted that the number of airstrikes and targets hit actually fell slightly in March, but said his group’s research indicated that civilian deaths had risen sixfold in Syria, with more than 350 killed last month alone.
“This indicates to us a possible loosening of U.S. battlefield rules,” he said, “which is placing civilians at greater risk of harm.”
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