U.S. authorities probing the shooting spree at Washington's Navy Yard, which left 13 people dead and eight injured, said Monday that a lone gunman carried out the killings.

"We don't have any evidence or indication at this stage that there was another shooter, even though we haven't completely ruled that out," said Washington mayor Vincent Gray.

The White House was locked down in a sudden security alert Monday after a man threw firecrackers over the outer fence, sparking alarm hours after a gun massacre in Washington.

A Secret Service source confirmed to Agence France Presse that the man set off firecrackers, discounting some earlier reports that shots were fired, in a incident that scrambled heavily armed security teams at the White House.

A U.S. envoy said Monday that Washington still hoped to talk directly with the Taliban to support an Afghan peace deal but that the militants seemed unwilling to do so.

The Taliban opened an office in the Qatari capital Doha in June aimed at talking to the United States ahead of next year's withdrawal of most American troops, but diplomacy collapsed before it even began.

A U.S. Navy commander told Monday how he saw a co-worker shot in the head just a few feet from him, during mass shooting on a Washington base that left at least 12 people dead.

Navy officer Tim Jirus told reporters the man from the Washington Navy Yard's maintenance department was felled by gunshots as workers scrambled out of buildings following reports of a shooter on the rampage.

A shooting rampage in the heart of a U.S. Navy complex in Washington left at least 12 people dead Monday. One shooter was killed, but police were looking for two other possible gunmen wearing military-style uniforms.

President Barack Obama said he is morning "yet another mass shooting" and called it a "cowardly act."

The United States said Monday it would welcome a plan for Syria to hand over its chemical weapons but expressed skepticism at the Russian initiative, which is designed to head off American air strikes.

However, the U.S. Senate announced later on Monday that it will hold a procedural vote Wednesday on a measure authorizing President Barack Obama to use military force against Syria.

The Obama administration is already planning "for every contingency" in case of any fallout from U.S. military strikes against the Syrian regime, a top White House official said Sunday, as a defense official said the U.S. could increase the scale of strikes as needed.

White House Chief of Staff Denis McDonough refused to be drawn on whether President Barack Obama would go ahead with any strikes if Congress refuses to give military action a green light, nor would he be drawn on any specifics.

Syria's President Bashar Assad has vowed that his country would emerge "victorious" in any confrontation with the United States and its allies, in comments published Thursday by Lebanese al-Akhbar newspaper.

Washington and other Western powers have warned of imminent military strikes against Assad's regime following a suspected chemical weapons attack that killed civilians, including dozens of children, in Damascus suburbs on August 21.

The U.S. on Thursday sidestepped questions about former Egyptian president Hosni Mubarak's release, saying it was a matter for Cairo to decide, but called for his ousted successor Mohammed Morsi to be freed.

"With respect to the Mubarak trial and decisions made, this is an internal Egyptian legal matter," said State Department spokeswoman Jen Psaki.

The United States on Wednesday demanded "immediate access" for United Nations inspectors to the site of an alleged chemical weapons attack by government forces on civilians in Syria.

"We are working urgently to gather additional information," said White House spokesman Josh Earnest in a statement expressing deep concern over the reports.