WASHINGTON (BBC) - Top Democrats in the U.S. Congress have warned President George W. Bush that he does not have the authority to go to war with Iran.

Washington is in dispute with Iran over its nuclear program, and senior U.S. officials have accused it of supplying weapons to Shia insurgents in Iraq.

The warning came ahead of a House of Representatives vote likely to condemn the recent surge of U.S. troops in Iraq.

The Senate is due to vote on the troop plan in an unusual Saturday session.

House of Representatives speaker Nancy Pelosi acknowledged Bush has said he wants a diplomatic solution to the rift with Iran, saying "I take him at his word".

But she also said that Congress should assert itself "and make it very clear that there is no previous authority for the president, any president, to go into Iran".