The United States has closed its embassy in Syria, warning that President Bashar al-Assad is increasing his campaign of violence against opponents following the Russian and Chinese veto of U.N. action against his government.

After more than one month of talks aimed at improving security around the U.S. embassy in Damascus, the Obama administration withdrew its last officials from Syria Monday, saying the Assad government “failed to respond adequately” to concerns about conditions around the facility.

The State Department says the deteriorating security situation makes clear what it calls “the dangerous path” President Assad has chosen, as well as his inability to fully control Syria.

With government forces continuing to bombarded Syria's third-largest city of Homs, U.S. officials say the international community must act without delay to support an Arab League plan for President Assad to step aside.

Russia and China Saturday vetoed a U.N. Security Council resolution supporting that plan. U.S. Deputy Assistant Secretary of State for Near Eastern Affairs Aaron Snipe says Moscow and Beijing are encouraging President Assad's campaign of violence.

"They voted on the side of a leader who is now further emboldened to commit heinous crimes against his people," said Snipe.

Snipe says the suspension of embassy operations and the withdrawal of Ambassador Robert Ford does not mean the United States is abandoning Syria.

“The door for diplomatic ties remains open with the Syrian people and with the Syrian government," he said. "Ambassador Ford still remains the United States ambassador to Syria and its people. And though he will be now based in Washington, he will continue engaging with the Syrian people and supporting the Syrian people in whatever way that we can.”

While in Damascus, Ford made use of social media to keep in touch with Syrians. U.S. officials say he will continue to maintain his Facebook page and Twitter account from Washington. The Polish embassy in Damascus will now provide consular services for the Americans who remain.