WASHINGTON (Reuters) - International accounting standards will make it harder for banks to keep assets off their balance sheets, a regulator said on Monday, as the United States mulls whether to broaden its use of foreign rules.

Under current U.S. accounting rules, companies can keep certain loans, such as those linked to risky mortgages and credit card debt, in off-balance sheet vehicles known as qualified special purpose entities (QSPEs).

The United Kingdom adheres to international financial reporting standards (IFRS), which are more flexible accounting rules but have forced firms to include more assets on their books.

"You might think it is advantageous to have a precise rule," said Paul Boyle, the chief executive of the Financial Reporting Council, an independent regulator of accounting and auditing standards.

"But if you have a precise rule it also makes it possible to design something that is precisely just outside the rule," Boyle said on the sidelines of a U.S. Securities and Exchange Commission event to discuss international accounting issues.

"Therefore the more principles-based approach under IFRS and what we have under UK (Generally Accepted Accounting Principles) makes it much more difficult to design something in such a way that it is off-balance sheet," he said.

Companies such as Deutsche Bank (DBKGn.DE: Quote, Profile, Research), which converted to international accounting standards have had to put about 200 off-balance sheet entities back on their books.

"You could not rely on a specific rule to give you a yes or no answer. You had to understand what was going on in the (off-balance sheet vehicles,)" Deutsche Bank managing director Charlotte Jones said at the accounting roundtable event.

Jones said that many of the vehicles that were brought back on the balance sheet were originally created using U.S. accounting rules and "a lot" were set up as QSPEs.