The talks are focused on Iran’s 20 percent enriched uranium, which is just below weapons grade. Iran has also offered increased access to international nuclear inspectors to its military site, to which Tehran has previously blocked access.
But the senators say that focusing on Iran’s highest-level uranium is not sufficient, because that has only been developed in the last two years.
“Focusing only on these recent manifestations of Iran's nuclear program, without also addressing older and broader enrichment and proliferation-sensitive activities, would effectively reward the Iranians for their escalation and allow them to move back the goal posts,” they wrote.
They also urged the Obama administration to keep up economic sanctions against Iran, which the administration has signaled it will do. The Senate passed new sanctions against Iran on Monday.
The senators concluded by returning to President Obama’s statements that all options must be on the table to prevent Iran from getting a nuclear weapon.
“The U.S. must be prepared, if necessary, to use military force to stop Iran from getting a nuclear-weapons capability,” they wrote.