Obama moving toward sending lethal arms to Syrian rebels, officials say President Obama is preparing to send lethal weaponry to the Syrian opposition and has taken steps to assert more aggressive U.S. leadership among allies and partners seeking the ouster of President Bashar al-Assad, according to senior administration officials. The officials emphasized that political negotiation remains the preferred option. To that end, the administration has launched a new effort to convince Russian President Vladimir Putin that the probable use of chemical weapons by the Syrian government - and the more direct outside intervention that could provoke - should lead him to reconsider his support of Assad. President wants to assert more aggressive U.S. leadership among allies seeking Assad's ouster. The second bombing in two days underscores the vulnerabilities of Syrian President Bashar al-Assad. Local bodies, many unelected, have been set up in rural north out of necessity, but lack resources. Walter Pincus Chemical weapons in Syria must be verified - and doubled-checked - before any U.S. action is taken. Colum Lynch and Abigail Hauslohner In Damascus, Prime Minister Wael al-Halki is reported unharmed in an attack that killed at least nine people. But Obama, who spoke by telephone with Putin on Monday and is sending Secretary of State John F. Kerry to Moscow in the coming days, is likely to make a final decision on the supply of arms within weeks, before a scheduled meeting with Putin in June, officials said. Confirmation of the use of chemical weapons by the government, Obama said Tuesday, would mean "there are some options that we might not otherwise exercise that we would strongly consider." At a news conference, he emphasized the need to "make sure I've got the facts. . . . If we end up rushing to judgment without hard, effective evidence, we can find ourselves in a position where we can't mobilize the international community to support" additional action. Administration officials have made repeated reference to the Bush administration's inaccurate claims of weapons of mass destruction to justify its invasion of Iraq. Yet even as Obama voiced caution in responding to what he has called the "red line" of chemical weapons, officials described him as ready to move on what one described as the "left-hand side" of a broad spectrum ranging from "arming the opposition to boots on the ground." "We're clearly on an upward trajectory," the official said. We've moved over to assistance that has a direct military purpose. Officials did not specify what U.S. equipment is under consideration, although the rebels have specifically requested antitank weapons and surface-to-air missiles. Insertion of U.S. troops is opposed by Syria's neighbors and by the American public in a series of recent polls. It remains highly unlikely barring a spillover of the conflict into major regional instability, significant chemical weapons use, or indications the weapons are falling into the hands of al-Qaeda-linked Islamist militants fighting alongside opposition forces. In lieu of U.S. troops, American and allied military and contract personnel have been training Jordanian and rebel forces to deal with the chemical weapons threat. U.S. intelligence has also tried to contact Syrian government units charged with protecting the weapons to warn against their use, and United Nations experts are preparing to secure chemical sites in the event of a negotiated cease-fire. But the senior official, one of several who discussed internal administration deliberations on the condition of anonymity, said Obama has "not closed the door to other military actions," in response to calls from the opposition, and some members of Congress, for protection against Syrian ballistic missiles and air attacks.