TOKYO (Reuters) - Shares in Japan's Toshiba Corp <6502.T> extended losses on Friday, weighed down by concern over a potentially bigger-than-expected $6 billion writedown triggered by cost overruns at its U.S. nuclear business.

The shares were down 6 percent in early trade and pared some losses to trade down around 4 percent by 0012 GMT. Toshiba's shares have almost halved since the group first announced in December the prospect of a nuclear writedown.

(Reporting by Clara Ferreira Marques; Editing by Himani Sarkar)