Iran has accused six major powers of "unreasonable behavior" over its disputed nuclear program, but the European Union said on Tuesday it would stick to a dual approach combining diplomacy with the threat of sanctions.

Tehran''s accusation was contained in a letter from its top nuclear negotiator, Saeed Jalili, that was delivered to European Union foreign policy chief Javier Solana on Monday.

"It''s a letter that in a way complains about our policy but our policy is clear. It''s a double-track approach," Solana told Reuters in Berlin on Tuesday.

Asked whether the letter would make negotiations with Iran more difficult, Solana replied: "It''s just a letter."

The United States and other Western powers suspect Tehran is seeking a nuclear bomb under cover of its civilian nuclear program. Iran, the world''s fourth-largest oil producer, denies it has any such intentions, saying it only wants to generate electricity.

Jalili''s letter, a copy of which was obtained by Reuters, was addressed through Solana to foreign ministers of the six-power group consisting of the United States, Britain, France, China, Russia and Germany.

In Washington, a U.S. official said the six powers planned to hold a conference call later this week to discuss the letter and their possible response, but declined to say exactly when.