Government testing for residues of glyphosate has been put on hold, slowing the U.S. Food and Drug Administration's (FDA) first-ever endeavor to get a handle on just how much of the controversial chemical is making its way into U.S. foods.

The FDA, the nation's chief food safety regulator, launched what it calls a "special assignment" earlier this year to analyze certain foods for residues of the weed killer after the agency was criticized by the U.S. Government Accountability Office for failing to include glyphosate in annual testing programs that look for many less-used pesticides. Glyphosate is the most widely used herbicide in the world and is the key ingredient in Monsanto's flagship Roundup.

Glyphosate is under particular scrutiny after the World Health Organization's cancer experts declared last year that the chemical is a probable human carcinogen. Several private groups and nonprofits have been doing their own testing and have been finding glyphosate residues in varying levels in a range of foods, raising consumer concerns about the pesticide's presence the American diet.

The FDA's residue testing for glyphosate was combined with a broader herbicides analysis program the agency set in motion in February of this year. But the glyphosate testing has been particularly challenging for the FDA. The agency was finally forced to put the glyphosate residue testing part of the work plan on hold amid confusion, disagreement and difficulties with establishing a standard methodology to use across the agency's multiple U.S. laboratories, according to FDA sources. Equipment issues have also been a problem, with some labs citing a need for more sensitive instruments, sources at the FDA said.

FDA spokeswoman Megan McSeveney confirmed the testing suspension and said the agency is not sure when it will resume.