BEIRUT, Lebanon — Syrian rebels accused the authorities on Tuesday of launching an airstrike on an olive press “filled with people” in fields just outside the northern city of Idlib, killing at least 20 people and wounding 50 as they waited to have their olives turned into oil.

Two activist groups, the Syrian Observatory for Human Rights, based in Britain, and the Local Coordination Committees, which rely on local activists for their reports, both said the strike exacted a heavy toll on civilians, making it only the latest in many such attacks that have caused casualties among noncombatants in Syria’s grinding civil war.

The authorities made no immediate comment on the claims, which came as rebel forces sought to secure a string of strategic gains, including a dam on the Euphrates River that they claimed to have overrun before dawn on Monday.

Ahmad Kadour, an antigovernment activist in Idlib who was reached by Skype, said a government warplane dropped two explosive devices onto the Abu Hilal oil press, where throngs of people could be seen. He said rebel forces were “doing everything in their power to rescue people.”