A Japanese woman has claimed she discovered fragments identified as "dental material" in a McDonald's hamburger, the company said Friday, the latest in a series of food contamination woes for the fast-food giant.

The claim comes two days after McDonald's held a press conference where it acknowledged several foreign objects had been found in its food, including a human tooth in a container of french fries at an outlet in Osaka.

In television footage aired Friday, the unidentified woman told the Asahi network that she found three tiny fragments of what looked like teeth in a burger she bought at a McDonald's in northern Kushiro city in September.

"I took a bite and there was a crunch," the woman said, adding she initially thought it was a piece of sand or stone.

"I can't help thinking that it was (already) in the meat," the customer added, as she showed a picture of the objects.

A third-party examination determined the opaque white fragments were "dental material", according to company spokeswoman Miwa Yamamoto, who added that the substance is commonly used to fill cavities or in other dental work.

McDonald's, however, would not confirm the woman's claim that the fragments were inside her burger.

Yamamoto said the customer was told that there was an "extremely low chance" it could have fallen into raw material given the highly-mechanised process.

None of the employees at the outlet had issues with their teeth at the time, and the customer denied it could have come from her own teeth, Yamamoto said.

On Wednesday, McDonald's acknowledged that a human tooth had been found in fries sold by another outlet last year, while it has also been hit by incidents in which pieces of vinyl were found in chicken nuggets and a tiny piece of hard plastic in a sundae.

Japanese media reports pointed to even more cases of contamination, including a piece of metal in a pancake.

The incidents mark another public relations setback for the firm, which is struggling to recover from the reputational battering it took in the summer when a Chinese supplier was found to be mixing out-of-date meat with fresh produce.

Then, late last year, the company had to airlift an emergency supply of french fries from the US after a chip shortage had resulted in rationing at its 3,000 restaurants across Japan.

Labour disputes on the US West Coast had bunged up the export chain, leaving Japanese firms scrambling to secure fresh supplies.

The difficulties looked set to hit McDonald's bottom line, with the Japanese unit earlier saying it was on track to report a 17 billion yen ($143 million) annual loss for 2014.