BEIRUT — Syrian rebels routinely kill captured soldiers and suspected regime informers, human rights monitors said Thursday, warning of mounting war crimes committed by those trying to topple President Bashar Assad.

Reports of rebel abuses come as the Syrian opposition appears to be gaining momentum in a 2-year-old conflict that, according to the U.N., has killed more than 70,000 people.

Abuses by the Assad regime remain far more deadly, systematic and widespread, particularly attacks on civilians with imprecise battlefield weapons, including widely banned cluster bombs, the London-based group Amnesty International said.