U.S. President Barack Obama argued for offering Iran “modest” relief on sanctions in exchange for progress on nuclear talks and urged Congress to hold off on imposing more economic penalties.

“We can dial those sanctions right back up” if Iran doesn’t live up to an agreement, Obama said yesterday at a White House news conference. “If we’re serious about pursuing diplomacy, then there’s no need for us to add new sanctions on top of the sanctions that are already very effective and that brought them to the table in the first place.”

Obama got backing today from Senator Dianne Feinstein, the California Democrat who is chairman of the Senate Intelligence Committee. New sanctions at this time “would not lead to a better deal,” she said in a statement. “It would lead to no deal at all.

‘‘I am baffled by the insistence of some senators to undermine’’ the negotiations between world powers and Iran, she said.

Feinstein’s comments were a departure from criticism by other U.S. lawmakers and objections by Israeli Prime Minister Benjamin Netanyahu to a potential deal between world powers and Iran in Geneva talks, which are scheduled to resume next week. Secretary of State John Kerry has asked Congress to pause in acting on legislation adding to sanctions.

‘‘At this time, I see no reason to let up the pressure,’’ Pennsylvania Democratic Senator Bob Casey said yesterday in a statement, reflecting the president’s challenge in gaining support even from his own party.

Maintain Pressure

Until Iran agrees to halt all uranium enrichment and take other measures demanded by the United Nations Security Council, he said, ‘‘I will urge my colleagues on the banking committee to move forward with a package of additional sanctions.’’

Representative Eric Cantor of Virginia, the No. 2 House Republican, called for moving forward on sanctions legislation. ‘‘We need to make sure we maintain that posture and a vote on a sanctions bill would help,’’ he said after a briefing with U.S. officials including Undersecretary of State Wendy Sherman.

Kerry said last week that negotiators in Geneva were close to an agreement that would grant Iran limited sanctions relief in return for curtailing key nuclear activities. The goal is a first-step accord that would keep Iran from getting closer to a nuclear weapons capability during subsequent talks toward a comprehensive agreement to guarantee that Iran’s activities are exclusively peaceful.

Nuclear Monitors

In Vienna, UN nuclear monitors reported yesterday that Iran has stopped installing machinery to boost production capability at its uranium-enrichment facility even as the stockpile of its most sensitive nuclear material grew to a record. The Persian Gulf nation also slowed construction of a heavy-water reactor at Arak, according to inspectors from the International Atomic Energy Agency.

Iran hasn’t given a reason for the slowdown, which diplomats interpreted as a positive signal from the government. The unilateral action could be reversed.

Netanyahu has advocated further increasing sanctions pressure on Iran until it agrees to irreversibly give up its uranium enrichment operations, which could be used to produce fuel for nuclear weapons if the nation were to break out of international monitoring. Israel also wants construction terminated on a reactor at Arak, which could provide plutonium, an alternative route to a weapon.

Military Alternative

Obama said yesterday that attempting to resolve the Iranian nuclear issues through negotiations is better than the alternative of military action.

‘‘No matter how good our military is, military options are always messy, are always difficult, always have unintended consequences,” Obama said.

Sanctions have reduced Iran’s oil revenue to “about $40 billion to $45 billion” a year, from as much as $120 billion a year, Kerry said yesterday on MSNBC. In addition, that money is frozen in banks under banking sanctions, he said. Iran was the sixth-largest oil producer in OPEC last month, according to data compiled by Bloomberg.

West Texas Intermediate crude headed for a sixth weekly decline, the longest stretch of losses in 15 years, as rising U.S. supplies countered speculation that the Federal Reserve will maintain stimulus of the economy.

The administration has been briefing U.S. lawmakers in the attempt to get them to hold off on additional sanctions. Some members of Congress and Netanyahu said that last week’s talks in Geneva gave Iran the promise of relief without forcing an end to Iran’s uranium enrichment.

‘Modest Relief’

“We would provide very modest relief” at the margins of sanctions while leaving in place core sanctions on oil, banking and finance, Obama said. That approach allows the U.S. “to test how serious are they,” while if Iran isn’t serious “we will have lost nothing.”

“Let’s see if this short-term Phase One deal can be completed to our satisfaction” that Iran is not advancing its program, he said. “We can buy some additional months” in terms of Iranian breakout capacity.

Representative Steny Hoyer of Maryland, the No. 2 House Democrat, said he would have “no problem” holding off on a sanctions bill until after the next round of Geneva talks. He stopped short of saying what he favors after that.

“The Iranians need to know that Congress is very determined to move on sanctions unless, in the very near term, there’s very substantial, credible progress,” Hoyer said after a briefing yesterday with administration officials.

Sanctions advocates including Israel say they worry that any easing may reduce the economic pressure on Iran for further concessions.

Iran is negotiating with a group known as the P5+1, made up of the five permanent members of the UN Security Council -- the U.S., U.K., France, Russia and China -- plus Germany.