(Reuters) - An invasive alien weed, silverleaf nightshade, is threatening cotton and wheat crops in Syria and Iraq and could spread to Lebanon and Jordan, the United Nations Food and Agriculture Organization (FAO) said on Tuesday.

More than 60 percent of the farmland in Syria, growing mainly cotton and wheat, has been infested with the weed, originally from the American tropics, which sucks nutrients from the soil and starves crops of water, the FAO said in a statement.

Olive groves have been affected by the weed and a similar mass infestation has been reported in northwest Iraq. The invasive plant has also been spotted at sites in Lebanon and Jordan, where it will spread if nothing is done, it said.

"This particular type of weed competes aggressively with crops for nutrients whilst its deep root system dries down soil moisture," Gualbert Gbehounou, FAO Weed Officer, said in the statement.

The weed, a relative of the tomato, probably arrived in the Middle East as a result of globalization of trade through seeds accidentally brought over in containers or bags of farm commodities, the FAO said.

The Rome-based agency said it has been working on a project to help farmers manage and prevent further spread of the weed in the four countries, as requested by their governments.

The FAO recommended that farmers should rotate regular crops with the fodder crop alfalfa, which covers the ground and competes with silverleaf nightshade to prevent the weed from producing new seeds and reduce amount of weed seed in the soil.

The FAO said it sought to encourage the countries to review their regulatory environments and work together to reinforce control of the weed at the national and regional levels.