By Courtney McBride and Michael R. Gordon

WASHINGTON -- The Trump administration's bid in the coming days to reimpose United Nations sanctions against Iran, on the heels of its failed effort last week to extend an arms embargo on its Middle East foe, risks isolating the U.S. diplomatically while doing little to curb Iran's access to weapons from Russia and China, analysts and diplomats said.

Using the veto it wields as a permanent member of the Security Council, the U.S. aims to restore U.N. sanctions that were in place before the 2015 Joint Comprehensive Plan of Action nuclear deal between Iran and six world powers. The Trump administration exited the accord in 2018.

Asked Wednesday about the U.S. ability to reinstate the sanctions absent backing from other Security Council members, Secretary of State Mike Pompeo said he expects all countries to uphold their obligations.

"These will be fully a valid, enforceable U.N. Security Council resolution," he said. "We have every expectation that they'll be enforced just like every other U.N. Security Council resolution that is in place."

Mr. Pompeo will meet with U.N. Secretary General Antonio Guterres on Thursday in New York, said a Guterres spokesman.

Analysts say that the U.S. move, known as the "snapback," could plunge the Security Council into a legal quagmire and escalate the debate between Washington and its allies over how best to curtail Tehran's nuclear program.

European nations have warned they wouldn't support a unilateral U.S. effort to reinstate the sanctions, as they want to preserve the Iran deal. Russia and China have already argued the U.S. has no legal standing to reimpose the sanctions and have indicated they would ignore them if the U.S. muscles them through the Security Council.

"My own sense is this is going to end in a whimper, not a bang," said Wendy Sherman, the chief negotiator on the Iran accord during the Obama administration. "There will be lots of back-of-the-room discussions to figure out how this will be managed, and at the end of the day, I think it will be apparent that other than the United States, the permanent members of the Security Council do not see the U.S. as a participant in the JCPOA, and thus does not have standing to snap back sanctions," she said, using the initials of the formal name for the Iran nuclear deal.

The Trump administration's strategy for reimposing sanctions is based on its interpretation of Security Council Resolution 2231, which endorsed the Iran accord, and the veto power the U.S. is given in the body under the U.N. charter.

To buttress its case, the U.S. has circulated a memo to members of the Security Council outlining the administration's argument that Washington retains the legal standing to unilaterally trigger a sanctions snapback.

"That right endures," states a six-page memo by the State Department's Office of Legal Adviser, which was reviewed by The Wall Street Journal.

The document argues that the U.S. "has an explicit right" to initiate a snapback, based on its inclusion among a group of "JCPOA participant states." The U.S. withdrawal from the Iran nuclear accord didn't alter its standing under the resolution, the memo states.

But the European Union's foreign policy chief, former Obama administration officials and John Bolton, President Trump's former national security adviser, assert that the Trump administration forfeited that right to invoke the snapback arrangement when it left the Iran accord.

"The agreement's backers argue that Washington, having withdrawn from the deal, has no standing to invoke its provisions," Mr. Bolton wrote in a recent op-ed piece for the Journal. "They're right."

Beyond the legal dispute, the U.S. has yet to articulate a feasible plan to enforce the sanctions if the overwhelming majority of the council differs with its interpretation.

The stage for the snapback dispute was set during the talks over the Iran accord when the U.S. and its partners agreed to a provision under which a ban on arms sales to and from Iran would lapse on Oct. 18, 2020.

As the expiration of the ban drew near, Trump administration officials argued it was vital to extend it to prevent Iran from importing arms to build up its military might and supporting proxies with its own weapons. U.S. officials also touted the support of Israel and Gulf nations, who are neither on the council nor party to the Iran agreement.

European officials worried an extension would lead to a Security Council clash that could unravel the nuclear accord, which Iran is still partly observing. To avoid one, European diplomats floated a number of compromises.

When the Trump administration pushed for a Security Council vote extending the ban, Russia and China voted against it and European nations abstained. Only the Dominican Republic, which Mr. Pompeo visited Sunday, voted with the U.S. on the 15-member council as a temporary member.

Following the U.S. notification of snapback plans, the other members of the Security Council have 10 days to offer a measure opposing the sanctions, which the U.S. could then veto. The sanctions would come back into effect 30 days after the U.S. notification.

"When sanctions are restored, Russia and China might sell weapons to Iran," said a senior Trump administration official. "But they would be violating their obligations as a permanent member of the Security Council and destabilizing the Middle East. That is much worse for them than it is for us."

Write to Courtney McBride at courtney.mcbride@wsj.com and Michael R. Gordon at michael.gordon@wsj.com