Thallium, which is tasteless and odourless, is difficult to detect

The two Soviet-born women, Marina Kovalevsky and daughter Yana, who emigrated to the US in the 80s, fell ill two weeks ago while on holiday.

They are thought to have been poisoned with thallium, a highly toxic metal which is tasteless and odourless.

Russian officials say their condition is now improving.

It was described as "fairly serious" on Monday.

Concern over such poisoning has been heightened since the death of Russian emigre Alexander Litvinenko in London last year after he ingested another highly toxic heavy metal, polonium, correspondents say.