WASHINGTON (AP) -- President Barack Obama signaled Tuesday he would consider U.S. military action against Syria if "hard, effective evidence" is found to bolster intelligence that chemical weapons have been used in the 2-year-old civil war. Among the potential options being readied for him: weapons and ammunition for the Syrian rebels. Despite such planning, Obama appealed for patience during a White House news conference, saying he needed more conclusive evidence about how and when chemical weapons detected by U.S. intelligence agencies were used and who deployed them. If those questions can be answered, Obama said he would consider actions the Pentagon and intelligence community have prepared for him in the event Syria has crossed his chemical weapons "red line." "There are options that are available to me that are on the shelf right now that we have not deployed," he told reporters packed into the White House briefing room. Beyond lethal aid to the rebels, several government agencies are also drafting plans for establishing a protective "no-fly zone" over Syria and for targeted missile strikes, according to officials familiar with the planning. However, the officials, who spoke only on condition of anonymity because they were not authorized to publicly discuss the internal deliberations, stressed that Obama had not yet decided to proceed on any of the plans. As Obama raised the prospect of deeper U.S. involvement, Hezbollah's leader said Tuesday that his Iranian-backed militant group stood ready to aid Syrian President Bashar Assad. And new violence in Syria hit the capital of Damascus, as a powerful bomb ripped through a bustling commercial district, killing at least 14 people. Mindful that any military intervention in the combustible Middle East would be complicated and dangerous, Obama hinted the U.S. would probably avoid taking action unilaterally. Part of the rationale for building a stronger chemical weapons case against Assad, Obama said, is to avoid being in a position "where we can't mobilize the international community to support what we do." Obama has resisted calls to expand U.S. assistance beyond the nonlethal aid the government is providing the rebels. That has frustrated some allies as well as some U.S. lawmakers, who say the deaths of 70,000 Syrians should warrant a more robust American response.