"Their date of discharge has not been determined but they are expected to remain in the hospital this weekend, " Cedars-Sinai hospital spokeswoman Sandy Van said.

The State Department asked Moscow on Thursday to investigate the suspected poisoning of the Kovalevskys with thallium, a highly toxic metal that can cause a slow, painful death. The two women were initially hospitalised in Moscow and returned to Los Angeles on Wednesday.

Russian media said Kovalevsky and her daughter emigrated to the US in the 1980s. On their recent trip, they arrived in Moscow in mid-February and became ill about a week later.