WASHINGTON — Leading U.S. senators say Iran must end all uranium enrichment, potentially putting them at odds with the Obama administration ahead of new nuclear talks.

In a letter to President Barack Obama, six Democrats and four Republicans support this week’s negotiations in Geneva. They also seek tougher sanctions on Iran and a convincing threat of U.S. military force.

But the most contentious demand concerns enrichment. The legislators including Sen. Bob Menendez, the Foreign Relations chairman, and Sen. John McCain insist Iran shouldn’t able to continue enriching uranium inside Iran.

Obama recently recognized Iran’s right to nuclear energy. But he hasn’t said enrichment is acceptable in Iran, as Tehran demands.

The U.S. and world powers suspect Iran is trying to develop nuclear weapons.

Iran says its program is for peaceful energy purposes.