WASHINGTON—In a rare public warning, the U.S. Securities and Exchange Commission on Monday told broker-dealers and other market participants to “remain vigilant to market and counterparty risk” amid heightened volatility and global uncertainties. The agency urged broker-dealers to closely monitor counterparty risk, collect “margin” or collateral from counterparties to “the fullest extent possible,” and stress test their positions. Global stock and commodities markets have been unusually volatile in the wake of Russia’s invasion of Ukraine and a slew of Western retaliatory sanctions on Russia’s financial system and its oil exports. Wild swings in the prices of oil, metals, and other raw materials last week generated more margin calls at clearing houses and trading firms, forcing counterparties out of the money to stump up liquid collateral they must pledge to secure their trades. Sudden, large margin calls can put financial stress on counterparties that do not hold sufficiently liquid assets. “Staff …