Story highlights President Barack Obama says now is not the time for new sanctions

A six-month interim agreement formally begins Monday

Iran must limit its nuclear program in exchange for sanctions relief

Talks will continue on a broader deal to prevent Iran from developing nuclear weapons

President Barack Obama gave an impassioned speech to Democratic senators gathered at the White House overnight, urging them that new sanctions could damage negotiations with Iran over its nuclear program.

"They discussed Iran and the President made the case for why new sanctions legislation now would hurt us," a senior administration official told CNN Senior White House Correspondent Jim Acosta.

A senator who was in the room Wednesday night called it "one of the most powerful arguments" about the issue he had ever heard from Obama.

Obama's statements during the closed-door session comes as the clock is ticking on an interim nuclear deal with Iran.

A six-month interim agreement formally begins Monday. That deal means that Iran must dismantle or freeze some of its nuclear program and open it to more international inspections in return for limited relief from crippling international sanctions.

Assuming everything goes as planned, further negotiations between Iran and the United States, France, Russia, China, Britain and Germany will seek a broader agreement intended to prevent Tehran from developing a nuclear weapon.

Meanwhile, pro-Israel members of Congress are seeking additional sanctions against Iran that would take effect if the talks break down.

Obama has said sanctions may mess up the pending negotiations.