Top U.S. diplomats are stridently pledging the Trump administration's determination to drive Iran out of Syria, even as it prepares to withdraw its military presence. And they're fielding new ideas to further that aim — even if it means withholding crucial aid. Doubling down on the Trump's administration's anti-Iran message, the senior policy advisor to Secretary of State Mike Pompeo, Brian Hook, vowed no U.S. reconstruction assistance to the war-torn country until Iranian forces and its proxies are completely driven out. "We think that if working with everybody in the region, and if we construct our diplomacy the way we want to, that we will be able to get rid of all forces under Iranian control," Hook told CNBC's Hadley Gamble in Abu Dhabi. "We are not going to permit them do in Syria what they did in Lebanon, and we will be withholding reconstruction assistance, we have a number of tools at our disposal to help accomplish that objective."

Hook's staunch message echoed that of senior administration officials Pompeo and national security advisor John Bolton, who have long held a hard line on Iran and until December had pledged a long-term U.S. presence in Syria with the aim of both countering the Islamic State and pushing back on Iranian military activity there. The policymakers have had to change tack since Trump's surprise announcement on December 19 to pull all U.S. forces out of Syria. Now, diplomats insist that while they plan to carry through Trump's decision, the mission to expel Iran from Syria remains unchanged. Critics of the withdrawal move warn that IS is not fully defeated, and that leaving so abruptly would mean abandoning local allies, namely Syrian Kurdish fighters, that have carried the brunt of the ground fight against IS alongside U.S. forces. Allies have been left confused as to the administration's policy goals in the region. At the same time, many senior officials in the Middle East and regional experts alike say that despite Washington's promises, the idea of Iran and its proxy forces actually leaving Syria is inconceivable. To that notion, Hook was defiant. "Well, we don't listen to the council of defeatism," he said. "If we always listened to our critics, we wouldn't be getting very far in foreign policy."