WASHINGTON - A Democratic US senator leading the charge to pass new sanctions on Iran despite objections from the Obama administration said on Friday the measure is a "diplomatic insurance policy" to push Tehran to comply with agreements to curtail its nuclear program.

Fifty-nine senators - 16 of them Democrats - of the 100 in the chamber were co-sponsoring the bill, despite the White House's insistence that it could imperil delicate international negotiations with the Islamic Republic.

Senator Robert Menendez, the chairman of the Senate Foreign Relations Committee, disputed that in an op-ed published in The Washington Post on Friday, saying the bill would bolster diplomacy, not threaten it.

"The proposed legislation is a clarifying action," he wrote.

"It allows all sides to negotiate in certainties and provides one year of space for the parties to continue talking. It spells out precisely the consequences should the agreement fail. This should motivate Iranians to negotiate honestly and seriously," he said.

Menendez is the main sponsor of the "Nuclear Free Iran Weapon Act," which would impose new sanctions on Tehran if it breaks an agreement to curb its nuclear program. The White House has threatened a veto, and Iran has said an interim nuclear agreement would be dead if Congress imposes new sanctions.

The bill would also place sanctions on Iran if it does not agree to a comprehensive deal later this year or next. The United States and five other world powers agreed to a six-month interim deal with Iran in Geneva in November that can be extended to a year.

White House spokesman Jay Carney said on Friday the administration still feels the proposed bill would be harmful.

"It could, if they were to do it, actually weaken the sanctions structure that's in place by undermining faith among our international partners and providing Iran the opportunity to say that we have been negotiating in bad faith," Carney said at a daily news briefing.

The 59 co-sponsors mean the bill is near the 60 votes needed to pass most legislation in the Senate and the 67 necessary to overcome a presidential veto. But there has been no indication from Democrat Harry Reid, the Senate Majority Leader, on when it would come to the floor for a vote.

Separately, US Representative Eliot Engel said he was "deeply troubled" about a report that Russia is negotiating an oil-for-goods swap with Iran, saying it raises questions about Moscow's commitment to the negotiations to end Iran's pursuit of nuclear weapons.

Reuters reported on Friday that the deal being negotiated is worth $1.5 billion a month, and would let Iran raise its oil exports by up to 500,000 barrels per day. US and European sanctions on Iran's disputed nuclear program have combined to cut Iran's oil exports by about 1 million bpd.

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