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Barley and livestock production are also being hit.
In addition to the worst drought since 2008, three years of civil war have ravaged infrastructure, leaving long-term damage to irrigation due to impaired pumps and canals, power failures and a lack of spare parts, the agency said.
This will have "long-lasting effects on Syria's agricultural production" even after peace is restored, WFP said.
The threat posed by drought means the number of Syrians in need of emergency rations could rise to 6.5 million, up from 4.2 million now, Byrs said.
The WFP, which reached a record 4.1 million people with rations in March, said on Monday that it had to cut the size of food parcels to hungry Syrians due to a shortage of funds from donors.
WFP says the operation in Syria is its biggest and most complex, costing more than $40 million a week.
The funding figure includes the feeding of 1.5 million of the 2.6 million registered Syrian refugees who have fled to neighboring countries, mainly Turkey, Lebanon, Jordan and Iraq.
"We can expect more refugees to leave if, on top of the conflict, they feel that their lives are in danger because there is no food. But it's hard to say obviously because they could also move to other parts of Syria," Fatoumata Lejeune-Kaba, spokeswoman of the U.N. Refugee Agency, told reporters.
Overall, the U.N. has received just 16 percent of the $2.2 billion sought for its aid operations inside Syria this year, with the United States the largest donor at $108 million, followed by the European Commission at $53.7 million and the United Arab Emirates at $50 million.