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WASHINGTON — With a vote on new sanctions against Iran only days away, the Obama administration is making the case to members of the United Nations Security Council that Iran has revived elements of its program to design nuclear weapons that American intelligence agencies previously concluded had gone dormant.

The classified intelligence briefings — some held in Washington for foreign ministers and foreign leaders as they visited in recent months, others in foreign capitals — have been part of a lobbying effort to secure votes for the sanctions, the fourth round since 2006. European and American officials expect the vote could come as early as Wednesday, and they say they believe the sanctions will pass 12 to 3, with Turkey, Brazil and Lebanon likely to vote against the sanctions.

The briefings, according to foreign diplomats and some American officials, amount to a tacit admission by the United States that it is gradually backing away from a 2007 National Intelligence Estimate. It is using new evidence to revise and in some cases reverse conclusions from that estimate, which came to the much disputed conclusion that while Iran had stepped up its production of nuclear fuel, its leadership had suspended its work on the devices and warhead designs needed to actually build a weapon.