** U.S.-listed shares of nuclear energy companies rise premarket

** President Donald Trump said late on Saturday U.S. forces struck Iran's three main nuclear sites, joining Israel's military campaign aimed at preventing Tehran from building an atomic weapon

** If the Middle East conflict escalates, it could lead to broader geopolitical instability and affect uranium supply from key producer Kazakhstan, prompting countries to look elsewhere to secure uranium

** Uranium stocks rise: Uranium Energy UEC.A up 1.7%, Centrus Energy LEU.A up 5.4% and Energy Fuels UUUU.A up 2.2%

** Small nuclear reactor developer NuScale Power SMR.N and nuclear technology company Nano Nuclear Energy NNE.O up 2.6% and 1.7%, respectively

** Sam Altman-backed nuclear startup Oklo OKLO.N up 2.7%

(Reporting by Sumit Saha in Bengaluru)

((Sumit.Saha@thomsonreuters.com;))