The Trump administration will seek a U.N. Security Council vote next week on a resolution to extend a 13-year-old arms embargo that expires in October -- while warning that the U.S. will go ahead unilaterally if the resolution fails.

"We have an objective to extend the arms embargo," Brian Hook, special envoy for Iran, told reporters in a press call Thursday morning. "That can be done the easy way or it can be done the hard way, but it’s going to be extended."

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The U.S. has been lobbying for months to get an extension of the Iran arms embargo, which expires in October as one of the conditions of the 2015 Obama-era Iran nuclear deal -- formally known as the Joint Comprehensive Plan of Action (JCPoA).

Secretary of State Mike Pompeo has warned that if the U.N. does not act to extend the embargo, then the U.S. will seek to “snap back” sanctions as it claims it is authorized to do as a still-participating member of U.N. Resolution 2231, which codified the Iran deal, which the U.S. left in 2018.

“The Council would make an absolute mockery of that mission if it allowed the number-one state sponsor of terrorism to buy and sell weapons freely,” Pompeo said in remarks Wednesday.

U.N. Ambassador Kelly Craft said Thursday that Iran’s behavior has worsened in recent years and warned that Iran would be able to purchase weapons and be able to sell them to countries such as Venezuela, Syria and Yemen.

But next week’s Security Council resolution is likely to fail. Even if the U.S. is able to drum up the nine votes for it to pass the chamber, it would likely face a veto from China and Russia.

“Russia and China can either stand with the world’s number one state sponsor of terrorism, which makes them co-cosponsors, or they can stand for safety and protection for the people of Iran, the Middle East and the rest of the world,” Craft said.

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She reiterated warnings that the U.S. was prepared to use all the diplomatic tools available, and said she had not seen a convincing argument for why the embargo should not be extended.

“I’ve not heard anyone on the Council that I have spoken to thus far give me a convincing peace and security argument that Iran should have more access to weapons,” she said.

Hook later said it would be “diplomatic malpractice” and a “reckless decision” for the international community to allow the embargo to expire. He cited a recent U.N. report that a missile used to hit a Saudi target was of Iranian origin and that weapons interdicted on the way to Yemen were on Iranian origin. He also noted that the expiration would allow Iran to buy fighter jets, attack choppers, tanks, submarines and missiles with a range of up to 300 km.

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“Iran has conducted terrorist attacks across five continents. Iran recently engaged in a war game with a decoy of a US Navy destroyer,” he said. “Iran continues to make the argument for why the arms embargo needs to be extended.”

Hours later, it was announced that Hook would be stepping down from his post and would be replaced by Elliott Abrams, the hawkish special representative to Venezuela.

Fox News' Ben Evansky contributed to this report.