U.S. says its warning appears to have averted Syrian chemical attack

U.S. Defence Minister James N. Mattis is seen during a press conference before the commemoration of the 70th anniversary of the Marshall Plan at the George C. Marshall Center in Garmisch-Partenkirchen, Germany June 28, 2017. REUTERS/Michaela Rehle

BRUSSELS/ISTANBUL (Reuters) - U.S. Defence Secretary Jim Mattis said on Wednesday that the Syrian government of President Bashar al-Assad appeared so far to have heeded a warning this week from Washington not to carry out a chemical weapons attack.

Russia, the Syrian government's main backer in the country's civil war, warned that it would respond proportionately if the United States took pre-emptive measures against Syrian forces to stop what the White House says could be a planned chemical attack.