Brian Hook, US special envoy for Iran, stepping down

US President Donald Trump's special envoy for Iran is leaving his post after two years, Secretary of State Mike Pompeo announced Thursday.

It is unclear why Brian Hook is leaving but Pompeo said it was the special envoy's decision after he achieved "historic results countering the Iranian regime."

"He successfully negotiated with the Iranians the release of Michael White and Xiyue Wang from prison. Special Representative Hook also served with distinction as the Director of Policy Planning and set into motion a range of new strategies that advanced the national security interests of the United States and our allies," Pompeo said in a statement.

Elliott Abrams, who has led State Department's efforts on Venezuela, will take Hook's place following a transition period. Abrams will maintain his role vis-a-vis Venezuela as well.

Hook had been a chief architect of the Trump administration's maximum pressure campaign, which has reimposed economic sanctions on Iran in a bid to bring the Islamic Republic back to the negotiating table after the US unilaterally exited in 2018 a nuclear pact Tehran struck with world powers.

Trump's effort has failed to bring Iran to talks, but has succeeded in repeatedly bringing tensions in the region to a fever pitch and having Iran take steps away from commitments under the 2015 deal.

The administration will introduce next week a resolution in the UN Security Council to extend an arms embargo mandated on Iran under the pact that is set to expire in October. But it is unclear if there will be sufficient support in the body, and Russia and China, which each wield veto powers, have stood in opposition to the plan.