May 21 (Reuters) - Canada's main stock index opened lower Thursday as concerns about U.S.-Iran tensions resurfaced after a Reuters report on Tehran's stance on uranium stockpiles.

At 9:30 a.m. ET, the Toronto Stock Exchange's S&P/TSX composite index .GSPTSE was down 0.3% at 34,031.08 points.

(Reporting by Tharuniyaa Lakshmi in Bengaluru; Editing by Jonathan Ananda)