Military force would have only limited effect in stopping Iran from developing nuclear weapons but must remain an option, the head of the Joint Chiefs of Staff said Monday. Teheran shows no signs of backing down in the standoff over what the United States and other countries say is its drive for a nuclear bomb, Adm. Mike Mullen, the top US military officer, told his staff in an annual assessment of the nation's risks and priorities. "My belief remains that political means are the best tools to attain regional security and that military force will have limited results," Mullen wrote. "However, should the president call for military options, we must have them ready." Iran denies that its nuclear program is aimed at producing a weapon. The Mideast nation says it is developing nuclear energy. In the past two or three years the United States had all but ruled out an attack on Iran's known nuclear facilities as too risky, because of the backlash it might unleash. "Most critically, Iran's internal unrest, unpredictable leadership and sponsorship of terrorism make it a regional and global concern," heightened by what Mullen called "its determined pursuit of nuclear weapons." Mullen and other military leaders have suggested that if Iran was determined to build a weapon, an attack would probably fail to completely stop that effort. Mullen has tried to dissuade Israel from launching its own attack on Iran, whose leaders have called for Israel's destruction. Mullen's annual review says nothing about what kind of military force he wants at hand, but any attack would presumably be done by air. President Barack Obama has set a rough deadline of the end of this year for Iran to respond to an offer of dialogue and to show that it will allay fears of weapons development. The Obama administration is working with allies to ready a new set of international economic sanctions on Iran for repeatedly defying international demands to halt questionable activities and come clean about the nature and extent of the program. On Monday, Republican Sen. John McCain of Arizona voiced support for attempting economic pressure against Iran before considering military action. "Sanctions have to be tried before we explore the last option," he said on ABC's "Good Morning America." But McCain, the top Republican on the Senate Armed Services Committee, wondered whether other countries such as Israel have the patience to see if sanctions will work. Mullen, the president's chief military adviser, had said separately on Sunday that he is worried about Iran's intentions and said the clock is running on Obama's offer of engagement. "I've said for a long time we don't need another conflict in that part of the world," he told reporters traveling with him on a visit to Afghanistan, Pakistan and Iraq. "I'm not predicting that would happen, but I think they've got to get to a position where they are a constructive force and not a destabilizing force." In his assessment released Monday, Mullen also wrote that the main effort in Afghanistan must be to push forces into the war zone quickly, including the shifting of some personnel from Iraq. His year-end message serves as general marching orders for the coming year for his large staff of planners and others. "Afghanistan has deteriorated in the last year, but reversing the Taliban 's momentum is achievable," Mullen wrote. As to Iraq, he said security improvements mean the planned US withdrawal can go ahead. "We must finish well in Iraq," he wrote. Mullen listed several other threats and concerns, including threats that aren't identified with a given country such as terrorism, piracy and cyber attacks. "The United States has given more thought and resources to the cyber threat," he wrote, "but "impeded progress here is a serious risk to our national security posture."