The White House is consulting with international allies to reconsider its options in Syria after United Nations observers pulled out of the country on Saturday.
"At this critical juncture, we are consulting with our international partners regarding next steps toward a Syrian-led political transition as called for in Security Council resolutions," White House spokesman Tommy Vietor said in a statement on the UN pullout.
However chief monitor General Robert Mood said the escalating violence in Syria has put UN officials in increasing danger and prevented them from carrying out their mission, Reuters reported on Saturday.
International observers have come under fire since arriving in the country.
In May, a UN convoy was hit by an improvised explosive device while traveling through the town of Khan-Sheikhoun in northern Syria.
That same month, pro-Assad forces, backed by government troops, reportedly slaughtered over 100 Syrian civilians in the village of Houla, located north of the opposition stronghold of Homs.
The violence by Assad's troops against anti-government forces and civilians has led the White House and the international community to admit the Annan peace deal has largely failed.
However, the administration remains committed to resolving the Syrian crisis through diplomatic means.
"We call again on the Syrian regime to uphold its commitments under the Annan plan, including the full implementation of a ceasefire," Vietor said.
The United States is seeking Russian support for a plan to depose Assad, based on the power transfer plan used to remove former Yemeni president Ali Abdullah Saleh.
President Obama plans to discuss that effort with Russian president Vladimir Putin during the Group of 20 meeting in Mexico next week.
But Moscow has continually balked at any such plan that would disrupt the balance of power in Syria, one of its strongest allies in the region.
On Friday, a Russian warship carrying a small contingent of troops was sent to Syria to secure the country's military base in Tartus, according to recent news reports.
Along with deploying troops, state-sponsored Russian defense firms also continue to supply arms and equipment to Assad's forces.