The White House on Tuesday vowed to continue to work with its international partners to increase pressure on the Syrian government.

"We've made clear that President Assad has lost legitimacy. He had the opportunity to lead the transition and he did not take it and he's lost legitimacy because of it," said White House spokesman Jay Carney.

Carney urged Syria to halt violence and pull its security forces from Hama and other cities so that "a genuine transition to democracy can take place."

On July 11, U.S. Ambassador to Syria Robert Ford paid a controversial visit to the central Syrian city of Hama, which has been gripped by anti-government protests. The visit apparently triggered attacks by some Syrians on the U.S. embassy and the ambassador's residence in Damascus.

On the same day, U.S. Secretary of State Hillary Clinton said that Syrian President Bashar Assad has lost his legitimacy and Washington has "absolutely nothing invested in him staying in power," in one of the toughest rhetorics by senior U.S. officials on Syria.

In a bid to step up pressure on Syria, U.S. President Barack Obama on July 12 criticized Assad for missing "opportunity after opportunity" to bring about real reforms.