WASHINGTON – The costs of enforcing a safe humanitarian area in northern Syria through the policing of a no-fly zone outweigh the benefits, senior Pentagon officials said on Wednesday.Defense officials responded to questioning on the matter at a Senate Armed Services Committee hearing, where the committee chairman, Senator John McCain (R-Arizona), expressed long-held support for the idea.The front-runner for the Democratic nomination for president, former secretary of state Hillary Rodham Clinton, has also called for a no-fly zone as recently as Sunday.Such a policy would require “substantial” ground forces and would put the US military at risk of a direct confrontation with Syrian regime and Russian forces, Defense Secretary Ashton Carter said. US Air Force General Paul Selva also noted that Syrian integrated air defense and radar systems were activated recently, and would pose additional obstacles to the plan.Those in favor of a no-fly zone argue the plan offers humanitarian as well as strategic benefit.Such a safe space, they say, would halt the refugee flow gripping Europe; Provide a respite to those internally displaced in the wartorn country; And would provide diplomats from the West with leverage in their negotiations with Russia and Iran over the future of Syria.Carter and Selva said that, because of the extensive costs of the plan and the likelihood it would increase conflict, they had not brought the proposal to US President Barack Obama with their endorsements. Carter also said the United States is prepared to deploy advisers and attack helicopters if requested by Iraq to help it “finish the job” of retaking the city of Ramadi from Islamic State.Carter’s remarks were the latest sign of US preparations to intensify its military involvement in the battle against the terrorist group.Islamic State captured Ramadi, a provincial capital just a short drive west of Baghdad, in May in its biggest conquest since last year, and retaking it would be a major victory for Iraq’s Prime Minister Haider al-Abadi.Carter, speaking at a Senate Armed Services Committee hearing, said it has taken a “frustratingly long time” for Iraqi security forces to claw back territory. But he pointed to significant gains, including recapturing the Anbar Operations Center on the northern bank of the Euphrates River in the past 24 hours.“The United States is prepared to assist the Iraqi Army with additional unique capabilities to help them finish the job, including attack helicopters and accompanying advisers, if circumstances dictate and if requested by Prime Minister Abadi,” Carter said.The United States has recently announced plans to deploy elite American military teams to Iraq and Syria and Carter told the Senate hearing that he was in touch with coalition partners to ask them to contribute special operations forces.But he expressed frustration with Sunni Arab allies, who he believed could make additional contributions in the effort against Islamic State, a Sunni extremist group.“I too wish that particularly the Sunni Arab nations of the Gulf would do more,” Carter said. Reuters contributed to this report.