Top US diplomat John Kerry arrived in Switzerland late Sunday for tough talks on Iran and Ukraine, and to push Moscow's foreign minister to ensure a "credible" investigation into the killing of opposition leader Boris Nemtsov.
Less than a week after accusing Russian officials of lying 'to my face" about the fighting in eastern Ukraine, Kerry will have likely tense discussions with Russian counterpart Sergei Lavrov.
He will also step up the pace of nuclear talks with Iran's top diplomat Mohammad Javad Zarif with a series of meetings planned over the next three days.
Earlier Sunday Kerry urged Russia to carry out a "thorough, transparent, real investigation" into the murder of Nemtsov, a Russian opposition leader who fiercely criticized President Vladimir Putin.
Nemtsov was shot dead in Moscow on Friday, just two days before he was due to lead a major anti-government rally.
From Switzerland, Kerry is expected to travel to Saudi Arabia to meet with King Salman before flying to London to join a conference of foreign ministers from the Gulf Cooperation Council.