Harris returned to Washington late on Saturday, the outlet said. Sullivan reportedly stayed behind to work on developing strategies for practical solutions to support Ukraine's nuclear, energy and food security.
Switzerland is hosting a high-level conference on Ukraine at the Buergenstock resort outside of Lucerne from June 15-16. Some 92 countries and 55 heads of state have confirmed their participation, as well as eight organizations, including the European Union, the European Council and the United Nations. Russia has not received an invitation, but even if it had, it would not attend the conference, Vladimir Khokhlov, the spokesman for the Russian Embassy in Bern, told Sputnik in April.