TOKYO, July 21 (Reuters) - Japanese Premier Shigeru Ishiba vowed to remain in his post on Monday after his ruling coalition suffered a bruising defeat in upper house elections, prompting some in his own party to doubt his leadership as the opposition weighed a no-confidence motion.

TOKYO — Japanese Prime Minister Shigeru Ishiba said Monday he will stay in office to tackle challenges such as rising prices and high U.S. tariffs after a weekend election defeat left his coalition with a minority in both parliamentary chambers and triggered calls for his resignation.

Japan’s shaky ruling coalition is likely to lose its majority in the upper house, exit polls showed after Sunday’s election, potentially heralding political turmoil as a tariff deadline with the United States looms.