CARACAS, Venezuela  The oil giant Exxon Mobil has won court orders freezing as much as $12 billion in petroleum assets controlled by Venezuela’s government in an escalation of a dispute over efforts by President Hugo Chávez to assert greater control over the country’s oil industry.

Venezuela’s dollar-denominated bonds suffered their steepest drop in six months on Thursday on concerns that Mr. Chávez’s government could face a protracted legal battle with Exxon, preventing the government from raising cash through the sale of refineries abroad if the economy here slows after years of torrid growth.

Investors are also increasingly concerned about the financial health of the national oil company, Petróleos de Venezuela, amid reports that its debt is ballooning as its output declines.

The oil company is the largest single source of revenue for Mr. Chávez’s government, financing an array of social welfare projects and foreign aid to leftist allies.