(Reuters) - The U.S.-led coalition fighting Islamic State militants has not taken enough precautions to avoid civilian casualties in Syria and has underestimated the impact of its operations on civilians, Amnesty International said on Tuesday.

"It's high time the U.S. authorities came clean about the full extent of the civilian damage caused by coalition attacks in Syria," Lynn Maalouf, deputy director for research at Amnesty International's Beirut regional office, said in a statement.

"We fear the U.S.-led coalition is significantly underestimating the harm caused to civilians in its operations in Syria," Maalouf added.

Amnesty International said as many as 300 civilians have been killed in 11 attacks conducted by the U.S.-led coalition since September 2014.

"Analysis of available evidence suggests that in each of these cases, coalition forces failed to take adequate precautions to minimize harm to civilians and damage to civilian objects," Maalouf said.

"Some of these attacks may constitute disproportionate or otherwise indiscriminate attacks," she said.

The Pentagon, which did not immediately respond to a request for comment, says it takes great care to minimize harm to civilians.

In July the Pentagon disclosed that air strikes against militant targets in Iraq and Syria had killed 14 civilians between July 28 last year and April 29 this year.

The U.S. military has in the past made similar disclosures of civilian deaths in its air strikes against militant groups in Iraq and Syria, and the disclosures typically follow investigations lasting weeks or months to determine the veracity of reports of civilian deaths.

(Reporting by Eric Beech and Idrees Ali in Washington; Editing by Sandra Maler)