(Adds details from State Department briefing, background)
By Lesley Wroughton
WASHINGTON, March 18 (Reuters) - The United States on Tuesday suspended operations of Syria's embassy in Washington and its consulates and told diplomats and staff who are not U.S. citizens or permanent residents to leave the country.
State Department officials said while embassy and consular activities were affected the United States was not severing diplomatic relations with Syria despite failed peace initiatives to end the three-year-old war.
U.S. special envoy for Syria Daniel Rubinstein said Syrian President Bashar al-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 as envoy was announced 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.
Syria announced on March 10 it would stop providing consular services in the United States.
Syria's conflict has now entered its fourth year with an estimated 150,000 people killed in the war and little sign that diplomatic initiatives to ease the crisis are working.
Efforts by the United States and Russia to broker a peace agreement have faded amid worsening relations between the two countries over Ukraine.
State Department spokeswoman Jen Psaki said Tuesday's suspension meant that embassy staff no longer had diplomatic status in the United States. Less than a dozen embassy personnel are affected, she added.
Diplomats and their families have until March 31 to leave the United States, while administrative staff will have until April 30 to shutter the mission, Psaki said. Syria's Ambassador to Washington left in 2011.
The United States has dismissed news reports that Syrian diplomats have been mistreated by U.S. authorities as "completely false."
"Despite our differences we have continued to meet our international obligations with respect to the treatment of Syrian diplomats," Psaki said.
The United States does not have an ambassador in Syria. Former Ambassador Robert Ford left the country in 2011 after the United States received "credible threats against his personal safety in Syria," the State Department said at the time. (Reporting by Lesley Wroughton; Editing by Bill Trott)