Oil prices rose above $100 per barrel on Monday following the failure of 21 hours of ceasefire talks between the United States and Iran over the weekend. The negotiations were aimed at ending the war that began in late February but concluded without resolution.

U.S. stock markets showed relatively modest reactions to the news. The S&P 500 remained virtually unchanged in morning trading after erasing an earlier decline, while the Dow Jones Industrial Average fell 250 points, or 0.5%. The Nasdaq composite gained 0.3%. These movements were less dramatic than the extreme volatility that has characterized financial markets since the war began.

Following the failed talks, President Donald Trump threatened to impose a blockade of the Strait of Hormuz, a critical shipping route for global oil supplies. The narrow waterway serves as a passage for much of the oil produced in the Persian Gulf region. Iran has previously restricted traffic through the strait, contributing to global oil price increases.

Iran responded to the blockade threat by warning that no ports in the Persian Gulf or Gulf of Oman would be safe. "Security in the Persian Gulf and the Sea of Oman is either for everyone or for NO ONE," Iranian military and Revolutionary Guards stated through state broadcasting.

Brent crude oil, the international benchmark, reached $100.18 per barrel, well above its pre-war price of approximately $70 but below its peak of $119 during the conflict. The price pulled back from an earlier high of nearly $104 reached Monday morning, rising roughly 5% overall.

International markets showed broader concerns, with stock indexes declining across much of Europe and Asia. Hong Kong's Hang Seng fell 0.9% and South Korea's Kospi dropped 0.9%. In the U.S., the housing market continued to face pressure from higher mortgage rates, with March sales of previously occupied homes falling short of economist expectations.