US seeks 'transparent, real' probe into Nemtsov murder: Kerry

US Secretary of State John Kerry today urged Russia to carry out a "thorough, transparent, real investigation" into the murder of Russian opposition leader Boris Nemtsov, a fierce critic of President Vladimir Putin.

Nemtsov was shot dead in Moscow on Friday, just two days before he was to have led a major opposition rally against the government.

"The bottom line is that we hope there will be a thorough, transparent, real investigation not just who actually fired the shots, but who if anyone may have ordered or instructed this or been behind this," said Kerry, speaking to ABC television's "This Week" program.

"Our hearts go out to the Russian people," he said. "This was a man who was deeply committed to a better relationship with the world."

Another leading figure in US politics, US Senator Dianne Feinstein, a former chairwoman of the Senate Intelligence Committee, went so far as to implicate Putin in Nemtsov's death.

Asked by CBS television if there is a connection between the Russian leader and the death of Nemtsov, Feinstein responded: "Oh, I think so. I think there is.

"Whether Putin authorized it, whether he didn't, whether he knew about it, whether it was his friends or some of his military doing this, we'll wait and see with the investigation," she said on the "Face the Nation" television program.

"But I think it comes at a very bad time," as an armistice between Russian backed rebels and Ukrainian forces "is just beginning to be observed," Feinstein added.

She expressed high regard for Nemtsov as "a distinguished dissident -- not your average person on the street, but someone who actually served in the cabinet of Boris Yeltsin and is very respected," she said.

"(He) spoke fluent English, was prepared to take on Russia with respect to the Ukraine and what Russia has really been doing there, and was shot down in cold blood."

Police and investigators in Russia said Nemtsov was shot down by unidentified assailants as he was walking with a woman along a bridge not far from the Kremlin.

Tens of thousands of people marched in central Moscow on Sunday in memory of Nemtsov, 55.

President Barack Obama late Saturday condemned what he called Nemtsov's "brutal" and "vicious" murder, and praised the opposition leader as a "tireless advocate for his country, seeking for his fellow Russian citizens the rights to which all people are entitled.