Two activist groups - the Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees - said the targeted factory is west of the city of Idlib. The LCC says at least 20 people were killed and many others wounded in the raid, while the Observatory said "tens were killed or wounded."
Both groups depend on a network of activists on the ground around the country.
President Bashar Assad's regime has been launching intense air raids on rebels in recent months, mostly in Idlib, the nearby province of Aleppo, Deir el-Zour to the east and suburbs of the capital Damascus.
The most recent air raids have killed hundreds of people, including eight children on Sunday in the village of Deir al-Asafir near the capital, Damascus.
"It is a sign of despair," said Hilal Khashan, a political science professor at the American University of Beirut. He added that the regime forces are overstretched, and the air force is being used in areas that the army cannot easily reach.
"This is mass punishment. The regime is striking at civilian areas to make the people pay a price for not standing against advancing opposition forces," 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, told The Associated Press by telephone that dozens of people had gathered to have their olives pressed when the warplanes struck, causing a large number of casualties.
It was not immediately clear why the olive press was targeted. "It was a massacre carried out by the regime," said al-Yassin.
"Now is the season to press oil," said al-Yassin, noting that many olive press factories are not functioning in the area because of the fighting in the region. A large number of people were at the one near the city of Idlib.
"Functioning olive press factories are packed with people these days," he said.
New York-based Human Rights Watch said that evidence has emerged that an airstrike using cluster bombs on the village of Deir al-Asafir near Damascus killed at least 11 children and wounded others on Sunday. Cluster bombs have been banned by most nations.
"This attack shows how cluster munitions kill without discriminating between civilians and military personnel," said Mary Wareham, arms division advocacy director at Human Rights Watch. "Due to the devastating harm caused to civilians, cluster bombs should not be used by anyone, anywhere, at any time."
The Observatory also reported heavy fighting on the southern edge of the strategic rebel-held town of Maaret al-Numan, captured from government troops last month.
The town is on the highway that links the capital, Damascus, with the northern city of Aleppo, Syria's largest, a commercial center that has been the scene of clashes between rebels and troops since July.
The Observatory and al-Yassin said air raids on Maaret al-Numan killed at least five rebels.
The LCC said a shelling fell on the Yarmouk Palestinian refugee camp Tuesday, killing a boy and wounding another.
Also, Syria's state-run TV said a car rigged with explosives went off in the Damascus suburb of Artouz, killing at least two people and wounding four.
Syria's conflict started in March 2011 as an uprising against Assad's regime, but quickly morphed into a civil war that has since killed more than 40,000 people, according to activists.
Assad's regime blames the revolt on a foreign conspiracy. It accuses Saudi Arabia and Qatar, along with the United States, other Western countries and Turkey of funding, training and arming the rebels, whom it calls terrorists.
On Tuesday, the pro-government daily Al-Watan published a list with names of 142 Arab and foreign "terrorists," whom it said were killed in Syria over the past months.
The paper said Syria submitted the list to the U.N. Security Council last month.
The list had names from 18 countries. It listed 47 from Saudi Arabia, 24 Libyans, 10 Tunisians, nine Egyptians, six Qataris and five Lebanese.
International journalists who visit rebel-held areas say foreign fighters are taking part in the battles against Assad's forces. Western officials say there is little doubt that Islamist extremists from outside Syria, some associated with the al-Qaida terror network, have made inroads in Syria as instability has spread.
(Copyright 2012 by The Associated Press. All Rights Reserved.)