Secretary of State John Kerry said the US had no intelligence on who was behind the shooting of Russian opposition politician Boris Nemtsov.
"The bottom line is we hope there will be a thorough, transparent, real investigation, not just of who actually fired the shots, but who, if anyone, may have ordered or instructed this or been behind this," he told ABC News' This Week.
US Secretary of State John Kerry has said that he and his European counterparts are agreed that the conflict in eastern Ukraine "will not end through military force".
He emphasised that there was "no split" between US and European policy towards Ukraine.
"We will stand together in support of Ukraine and in defence of the common understanding that international borders ... cannot be changed by force in Europe or anywhere else." Mr Kerry added.
John Kerry says that the largest threat to Ukraine is Russian aggression but that the west is not seeking conflict with Russia.
The Secretary of State says the US is seeking a diplomatic solution to the conflict in Ukraine but is not prepared to "close our eyes" to Russian tanks and fighters crossing the border.
Speaking after talks in Kiev with Ukrainian President Petro Poroshenko, Kerry demanded an immediate commitment by Moscow to a ceasefire in eastern Ukraine that was backed up by actions on the ground.
Russia has consistently rejected accusations from Kiev and the West that it supports a pro-Russian rebellion in eastern Ukraine with troops and heavy weaponry.
The US has pledged $16.4 million in humanitarian aid to help civilians in eastern Ukraine.
Secretary of State John Kerry arrived in Kiev this morning for talks with the embattled Ukrainian leadership.
US Secretary of State John Kerry has written on Twitter:
US Secretary of State John Kerry has said that the insurgency by Islamic State militants in northern Iraq and Syria demands a "much fuller response" than just airstrikes.
Writing in the New York Times, he said:
Airstrikes alone won’t defeat this enemy. A much fuller response is demanded from the world. We need to support Iraqi forces and the moderate Syrian opposition, who are facing ISIS on the front lines ... In this battle, there is a role for almost every country. Some will provide military assistance, direct and indirect. Some will provide desperately needed humanitarian assistance for the millions who have been displaced and victimized across the region. Others will help restore not just shattered economies but broken trust among neighbors.
US secretary of state John Kerry warned today that the violent acts carried out by Islamist militants in Iraq "show all the warning signs of genocide"
"With the potential of further executions taking place," he said, "and because people are a minority huddled for safety on a mountain top, the United States has made its decision that it must save these lives.
"The world needs to join us in the condemnation of these actions," Mr Kerry added, speaking in the Afghan capital of Kabul.
US Secretary of State John Kerry has arrived in Vienna for talks with foreign ministers about Tehran's nuclear program.
Iran and the six powers- Britain, the US, France, Germany, Russia and China- aim to reach a long-term deal to end the decade-old nuclear standoff by July 20.
US Secretary of State John Kerry has said all of the reports of fraud and irregularities in the second round of the Afghan presidential vote need to be resolved.
The election run-off between the leading contenders, Abdullah Abdullah and Ashraf Ghani, has been deadlocked since the June 14 vote, raising concerns in Washington about a smooth transition of power in Afghanistan just as US troops are withdrawing.
Abdullah Abdullah has declared he was the winner of last month's run-off vote, despite preliminary results showing that Ashraf Ghani won the second round.
US Secretary of State John Kerry will travel to Vienna this weekend to try to break the deadlock in negotiations over Iran's nuclear programme, a senior Western official told Reuters.
The source, who spoke on condition of anonymity, said: "I can tell you that Kerry will be in Vienna this weekend, probably on Saturday."
However, the official added it was unlikely there would be an agreement signed in the near future, given the disagreements between Iran and western nations over Tehran's nuclear programme.