"Negotiations should continue so that we can reach possibly a worsening of the sanctions regime," said French presidential spokesman David Martinon.
"We need to increase pressure on Iran and the only way to do that is sanctions," he added. "For us, the sooner the better."
U.S. intelligence agencies, in a report released on Monday, concluded that Iran had a covert nuclear weapons program, but halted it in 2003. But Martinon said the report did not allay France's concerns. French President Nicolas Sarkozy and U.S. President George W. Bush discussed the report earlier this week.
"We cannot at all conclude that the threat has decreased," said Martinon.
He urged Iran to cooperate fully with the International Atomic Energy Agency and suspend uranium enrichment.
"The demands of the international community are more pertinent than ever," he said.
The discovery of the submarine has unveiled a few "inconsistencies." For example, how can one explain the fact that the sub was found where it needed to be searched for from the start?