WASHINGTON, July 23 - President Barack Obama's new trade enforcement unit is initially focusing on obstacles to U.S. exports in at least six countries, including Chinese industrial policies believed to violate world trade rules, a top U.S. trade official said on Monday.

"We're looking at problems in at least a half-dozen countries, and that number is going to grow. We've got problems around the world," Tim Reif, general counsel in the U.S. Trade Representative's office, said in a speech.

"At the same time, obviously, China presents a special challenge," Reif said.

Obama, who is in a tough race for reelection against Republican presidential challenger Mitt Romney, created the trade enforcement unit this year to make sure other countries live up to international trade obligations.

It draws on resources from throughout the federal government to bolster trade enforcement efforts led by Reif in the U.S. Trade Representative's office.

Romney has accused Obama of not being tough enough on China and has promised to use the threat of sanctions to get Beijing to change its trade practices.

The administration could file one or more new trade cases at the World Trade Organization before the November election, Reif told reporters after the speech, but he declined to discuss specific countries or issues.

Two factors that the administration considers in deciding whether to file a case is the economic cost of a particular problem and the precedent the ruling would set, he told the Center for Strategic and International Studies.

Reif said the Obama administration was pursuing a three-part enforcement strategy against China that includes "going after aspects of China's industrial policies that we can determine do not meet WTO standards," Reif said.

Examples include a case the United States brought against China's restrictions on exports of rare earths used in many clean energy and consumer electronics products, and another case it won against Chinese barriers to the country's trillion-dollar electronic payments market, he said.

Washington is also aggressively challenging Beijing's "newfound affinity" to use its countervailing and anti-dumping duty laws to retaliate against other countries that legitimately impose such duties on Chinese goods, he said.

The United States has filed three complaints in this area, including a case it won against Chinese duties imposed on a specialty steel product and a newer case against Chinese duties on U.S. auto exports, Reif said.

The Obama administration is also defending its use of countervailing and anti-dumping duties on certain Chinese goods against China's attacks on their legitimacy, he said.

WTO rules allow countries to impose anti-dumping duties on goods that evidence shows are unfairly priced and countervailing duties on goods that have benefited from government subsidies.