BEIJING, July 7 (Reuters) - China's drug safety watchdog has suspended sales of a drug used to treat acute leukemia and rheumatoid arthritis, Xinhua news agency said on Saturday, the latest in a string of food and drug security scandals.

China has faced sharp international criticism, especially from the United States, for failing to stop exports of toxic medical ingredients, foods and toys, and China's own national leadership has demanded stricter food and medicine standards.

Sales of methotrexate produced by Shanghai Hualian Pharmaceutical Co. Ltd. have been suspended because it caused an adverse reaction in several child leukemia patients in three hospitals in Shanghai and the southern province of Guangxi, Xinhua quoted the State Food and Drug Administration as saying.

Some children felt pain in their legs while others experienced difficulty in walking after being injected with methotrexate numbered 070403A and 070403B, it said.

The watchdog has ordered the Shanghai and Guangxi food and drug administrations to re-evaluate methotrexate, Xinhua said.

Xinhua quoted World Health Organisation statistics saying there have been 12,502 reports of adverse reactions to methotrexate worldwide. It did not give further details.

The food security issue burst into the international spotlight after tainted additives exported from China contaminated pet food in North America.