Eight mainly American volunteers were working with the farmers in an olive grove near the Palestinian village of Qusra when settlers came after them, said David Hummel, an American-German in the group.
"We were standing there peacefully, not a threat to anyone, when they started coming towards us and pushing us down the path," Hummel told AFP.
"They started attacking and beating us all with sticks and metal pipes and they were throwing rocks as well at us," he said.
"I was attacked on my legs, on my arms and here on my jaw as well and it was ... very violent," added the volunteer, showing bruising to his face.
Attacks on Palestinians in the West Bank have increased since the Gaza war erupted on October 7. Tensions have been further fuelled by an International Court of Justice ruling on Friday that Israel's occupation of Palestinian territories since 1967 was illegal.
The volunteers are from the International Solidarity Movement, a group that says it deploys people to form a "protective presence" for Palestinians at risk of facing violence in the West Bank.
Two women were among four activists treated at Rafidia hospital in the nearby city of Nablus, according to Qusra's mayor Hani Odeh. An AFP journalist saw at least three being treated in the hospital.
Israeli troops arrived and fired warning shots in the air to chase away the volunteers and farmers, according to the mayor.
The army said in a statement that "a number of masked Israeli civilians assaulted a group of foreign citizens while they were planting trees in the area of Qusra" and that "several" needed treatment.
"Soldiers were dispatched to the scene and fired warning shots into the air, causing the Israeli civilians to flee the area," they added, condemning any "acts of violence".
The volunteers from the ISM, which says it was set up "to resist the Israeli occupation of Palestinian land", have been in Qusra for about a month, according to the mayor.
The farmers wanted to "clear the land after settlers burned it some time ago", said Odeh.
About 10 people from the nearby Esh Kodesh Israeli settlement arrived to confront the farmers and foreign activists, he added.
Hummel said the group of settlers who attacked them included six women.
Several ISM volunteers have been injured in the West Bank and Gaza since it started work. Some have also been arrested.
Since October 7, at least 579 Palestinians have been killed in violence with settlers or Israeli troops, according to the Palestinian authorities.
At least 16 Israelis, including soldiers, have been killed in attacks involving Palestinians, according to official Israeli figures.
