WASHINGTON — The United States on Tuesday suspended operations of the Syrian Embassy in Washington and its consulates and told diplomats and staff who are not U.S. citizens or permanent residents to leave the country.

U.S. special envoy for Syria Daniel Rubinstein said Syrian President Bashar Assad had refused to step down and was responsible for atrocities against Syrians.

"We have determined it is unacceptable for individuals appointed by that regime to conduct diplomatic or consular operations in the United States," said Rubinstein, whose appointment was announced by the State Department on Monday.

"Consequently, the United States notified the Syrian government today that it must immediately suspend operations of its embassy in Washington, D.C., and its honorary consulates in Troy, Michigan, and Houston, Texas," he said in a statement.

The announcement comes as the Syrian conflict enters its fourth year, with an estimated 150,000 people killed by the war and little sign that initiatives to ease the crisis are working.

Efforts by the United States and Russia to broker a peace settlement now appear to have faded further amid worsening tensions between Moscow and Washington over Ukraine.