USA -backed forces cleared mines from the eastern fringe of the Islamic State's Syrian stronghold of Raqqa on Tuesday as a second front south of the city appeared to falter.
"The imperative to fight terrorism must not, however, be undertaken at the expense of civilians who unwillingly find themselves living in areas where ISIL is present", he added.
America is backing a coalition of Kurdish and Arab fighters which have begun a siege of the city to free it from the clutches of Islamic State.
Syria's vast deserts have become the main theatre of war in recent weeks as rival forces race to capture ground from the jihadist group, which is slowly retreating on several fronts.
United States forces are providing air support for the ground troops, and have been conducting airstrikes on the terror group's stronghold.
A video grab from the media arm of the Islamic State shows damage caused by air strikes in Raqqa city. The pro-government forces brought in additional equipment and called in airstrikes to repel the assault, and the USA was informed of those developments through the so-called "deconfliction line" it has set up with Russian military officials. Worldwide law prohibits its use in civilian areas because of its indiscriminate effects, from starting fires to causing excruciating burns for bystanders, according to Human Rights Watch.
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The Syrian observatory also quoted residents of the old city saying that the area was under heavy bombardment.
They urged parties to the fighting to take "all feasible precautions to avoid civilian casualties", as well as respect detainee rights and provide safe passage for fleeing civilians.
US defense officials said the weapon fired by the drone did not detonate and no one was hurt. It also would provide US -backed forces with an important bargaining chip to use in the event of a final peace settlement for Syria.
The U.S. military refused to comment on specific allegations after last week's attack in Raqqa but said it uses white phosphorous rounds "in accordance with the law of armed conflict.in a way that fully considers the possible incidental effects on civilians and civilian structures". In neighboring Iraq, the Islamic State is close to losing its onetime stronghold of Mosul, after a months-long battle led by US -trained elite units of the Iraqi army.
Last week, USA warplanes downed a pro-government drone that fired at a patrol unit of US -backed forces in southern Syria.