U.S. Secretary of State Marco Rubio said that Iran is nearing nuclear capability as the U.S. weighs diplomacy against mounting pressure.

President Donald Trump on Friday said Iran had a U.S. proposal about its nuclear program and knows it needs to move quickly to resolve a decades-long dispute, but Tehran denied receiving one yet.

Envoys for Iran and the United States have engaged in four rounds of talks. But exactly what a deal would look like remained unclear.