Leaders are expected to put pressure on Russia to end its support of Syrian President Bashar Assad.
After landing in Italy, Secretary Tillerson said the United States is rededicating itself to hold "any and all" who commit crimes against innocent people accountable. He made the remarks during a visit to a site of Nazi atrocities in Italy during World War II.
President Donald Trump ordered an air strike last week after nerve gas was used by Assad's forces in an attack that killed more than 80 people.
Italian Foreign Minister Angelino Alfano says Europe is offering "broad support" to the U.S. following the military response.
This is Secretary Tillerson's most high-profile trip as Secretary of State. After meeting in Italy he is expected to meet with Russian Foreign Minister Sergey Lavrov and foreign ministers from the U.K. and France.
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