The FAO said that figure included 1.5 million Syrians who "need urgent and immediate food assistance over the next three to six months, especially in areas that have seen the greatest conflict and population displacement."
"Close to a million people need crop and livestock assistance such as seeds, food for animals, fuel and repair of irrigation pumps," said the joint statement by the FAO and the UN's World Food Programme (WFP).
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"Further scaling up of food and livelihoods assistance will be required over the next 12 months as the people needing nutritional support are expected to reach three million," the statement said.
The survey, conducted in June by the FAO, WFP and the Syrian agriculture ministry, found that the farming sector has lost a total of $1.8 billion (1.5 billion euros) this year due to the conflict.
The losses included damages to crops such as wheat, barley, cherries, olive trees and vegetables, as well as to livestock and irrigation systems.
"While the economic implications of these losses are quite grave, the humanitarian implications are far more pressing," WFP's representative in Syria, Muhannad Hadi, said in the statement.
"The effects of these major losses are first, and most viciously, felt by the poorest in the country," it said.
"Most of the vulnerable families the mission visited reported less income and more expenditure -- their lives becoming more difficult by the day."
Farmers have been forced to either abandon farming or leave crops unattended due to the lack of labour, the scarcity of fuel and the rise in fuel costs, and insecurity, as well as power cuts, the report found.
Wheat harvests have been delayed in Daar'a, Homs and Hama as well as in the region around Damascus.
The report also said that deforestation was on the rise as farmers used the forests for fire wood due to the lack of cooking gas and fuel.
The WFP began sending food aid to Syria in October 2011 and has gradually scaled up its operations, reaching 540,000 people in July. It said it aims to reach 850,000 people this month.
The WFP said it faced a funding shortfall of around $62 million on an overall budget of $103 million.
The FAO began its assistance operations in December 2011 to more than 9,000 small herders and farmers' households.
The FAP said it needs $38 million for the next six months to help a total of 112,500 rural households
