BEIRUT — Syrian warplanes bombed an olive-oil factory packed with farmers Tuesday, killing at least 20 people in the latest regime strike to rip through a crowd of civilians, activists said.

The bombing comes as the civil war takes a devastating toll on an already beleaguered population. Human Rights Watch said it found “compelling evidence” that the regimeused cluster bombs in an airstrike that killed at least 11 children earlier this week.

It was not immediately clear whether the olive press was theintended target, or whether the plane misfired. The government generally does not comment on rebel claims and there was no official reaction to the latest allegations.

But two anti-regime activist groups - the Britainbased Syrian Observatory for Human Rights and the Local Coordination Committees - said the factory was hit Tuesday near the northern city of Idlib.

The Syrian Observatory for Human Rights said “tens were killed or wounded,” while the Local Coordination Committees said at least 20 people were killed. Syria restricts independent media coverage, making it difficult to determine the exact toll.

Both groups depend on a network of activists around the country.

President Bashar Assad’s regime has been relying on air power in recent months, mostly in the northern province of Idlib, the nearby province of Aleppo, Deir el-Zour to the east and suburbs of the capital, Damascus.

Hilal Khashan, a political science professor at the American University of Beirut, said the air force is being used in areas that the overstretched army cannot easily reach.

“This is mass punishment,” Khashan said. “The regime is desperate and wants to make the price of its opponents’ victory costly.”

Olive oil is a main staple in Syria. Tens of thousands of tons are produced annually.

Fadi al-Yassin, an activist based in Idlib, said by telephone that dozens of people had gathered to have their olives pressed when the warplanes struck, causing a large number of casualties.

“Now is the season to press oil,” said al-Yassin, noting that many olive press factories are not functioning because of the fighting in the region. “Functioning olive press factories are packed with people these days.”

Also Tuesday, Syria’s airforce targeted a village in northeastern Hasekah province as well as the town of Harim, in Idlib province, according to Turkey’s state-run Anadolu agency.

In other developments, NATO officers arrived in Turkey to select missile sites to counter Assad’s forces.

U.S., Dutch and German officers representing the three NATO countries with Patriot surface-to-air missile batteries visited Turkish provinces near the Syrian border Tuesday, authorities said.

Turkey’s military said Monday that the Patriots were a purely defensive measure and won’t be used to enforce a “nofly zone” or to launch attacks.

The conflict in Syria started 20 months ago as an uprising against Assad, whose family has ruled Syria for four decades. The conflict quicklymorphed into a civil war, with rebels taking up arms to fight back against a bloody crackdown by the government. According to activists, some 40,000 people have been killed since March 2011.

Assad blames the revolt on a conspiracy to destroy Syria, saying the uprising is being driven by foreign terrorists - not Syrians seeking change. On Tuesday, the progovernment daily Al-Watan published a list with names of 142 Arab and foreign terrorists it said were killed in Syria in recent months.

The list had names from 18 countries, including 47 from Saudi Arabia, 24 Libyans, 10 Tunisians, nine Egyptians, six Qataris and five Lebanese.

Analysts say most of those fighting Assad’s regime are ordinary Syrians and soldiers who have defected, havingbecome fed up with the authoritarian government. But increasingly, foreign fighters and those adhering to an extremist Islamist ideology are turning up on the front lines. The rebels try to play down their influence for fear of alienating Western support.

The regime, however, points to foreign fighters as evidence that the uprising is illegitimate.

As the conflict grinds on, however, the toll on civilians is growing. Winter is approaching, and temperatures can drop below freezing in northern Syria, where it often rains heavily. The parts of the country outside government control have to rely on smuggled supplies of gasoline and heating oil, which have already tripled in price.

New York-based Human Rights Watch said evidence has emerged that an airstrike using cluster bombs Sunday on the village of Deir al-Asafir near Damascus killed at least 11 children and wounded others.

Cluster bombs open in flight, scattering smaller bomblets over a wide area. Many of the bomblets don’t explode immediately, posing a threat to civilians long afterward. They have been banned by most nations.

Information for this article was contributed by Mehmet Guzel of The Associated Press and by Selcan Hacaoglu, Brian Parkin, Ali Berat Meric and Dana El Baltaji of Bloomberg News.

Front Section, Pages 6 on 11/28/2012