America's drugs watchdog believes that Chinese-made ingredients for a blood-thinning drug may have been deliberately contaminated.

The US Food and Drug Administration (FDA) said earlier that a chemical contaminant had been found in some batches of the drug heparin.

It has linked the contaminant to hundreds of severe allergic reactions and dozens of deaths across the US.

An FDA director said when and how the toxin had been introduced was unclear.

"FDA's working hypothesis is that this was intentional contamination, but this is not yet proven," Director of the FDA's Center for Drug Evaluation and Research Janet Woodcock told a congressional