Germany and France have reinforced a call for Iran to respond to concerns about its nuclear program in September or face tougher sanctions.

French President Nicolas Sarkozy says "initiatives must be taken during the month of September which take account of Iran's will or otherwise to cooperate." He says that, if it does not, "Germany and France will be united in calling for a strengthening of sanctions."

He says international agreement on further sanctions is important.

German Chancellor Angela Merkel said Monday after meeting Sarkozy: "Iran should know that we mean this very seriously."

The German government says officials from six countries trying to address concerns about Iran's nuclear program will meet Wednesday in Germany.