WASHINGTON – President Obama is preparing to send 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 Assad, according to senior administration officials.

The officials emphasized that supplying arms is one of several options under consideration and that political negotiation remains the preferred option. To that end, the administration has launched an 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.

But Obama, who spoke with Putin on Monday and is sending Secretary of State John Kerry to Moscow in the coming days, is likely to decide on the supply of arms to the opposition within weeks, before a scheduled meeting with Putin in June, officials said.

Confirmation of the use of chemical weapons by the Assad government, Obama said Tuesday, would mean that "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 repeatedly referred to the George W. Bush administration's inaccurate claims of weapons of mass destruction to justify its 2003 invasion of Iraq.

Yet even as Obama voiced caution in responding to what he has called the "red line" on chemical weapons, officials described him as ready to move on what one described as the "left-hand side" of a broad spectrum that ranged from "arming the opposition to boots on the ground."

"We're clearly on an upward trajectory," the senior 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.

American and allied military and contract personnel have been training Jordanian and rebel forces to deal with the chemical weapons threat. U.S. intelligence also has tried to contact Syrian government units charged with protecting the weapons to warn against their use, and U.N. 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 airstrikes.

Asked about the possibility of establishing a no-fly zone over rebel-held areas in Syria, the official said the administration was "reviewing all options."

Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, on Tuesday repeated his long-held reservations about a no-fly zone, emphasizing that it is more complicated and riskier than advocates believe. "I have to assume … that a potential adversary is not just going to sit back" and allow its air defense systems to be destroyed, Dempsey said at a luncheon.