The US Secretary of State is warning Russia to de-escalate the crisis in Ukraine. John Kerry was speaking in Kiev where he has been meeting with political leaders.
Mr. Kerry said the US and its partners would isolate Russia if it does not pull back from the Crimean peninsula. Vladimir Putin has denied that troops surrounding Ukrainian barracks in Crimea belong to the Russian army.
But Mr. Kerry says the claims by the Russian President are not believable.
Ukrainian and Russian Ministers have started holding talks in an effort to end the political crisis through peaceful means.
The move was announced by Ukraine Prime Minister Arseniy Yatsenyuk. Meanwhile the Russian President has spoken for the first time since the crisis began.
Vladimir Putin warned he would use "all means" to protect Russian citizens against what he called an "unconstitutional coup" in Ukraine.
He said deposed president Viktor Yanukovych fulfilled all conditions of a February 21st agreement with the Ukrainian opposition and effectively "gave up power".
Mr. Putin said Mr. Yanukovych is still the country's legitimate president and added Russia would "of course" provide financial aid to the southern region of Crimea.
Vladimir Putin has described the situation in Ukraine as an "unconstitutional coup"
He told a news conference at his residence outside Moscow that there was no need to use force in Crimea, but Russia reserved the right to do so as a "last resort".
Any intervention, he said, would be "legitimate and within the framework of international law".
And he claimed that Mr. Yanukovych "would have been murdered" if he remained in Ukraine during the unrest and no longer has a political future in the country.
Speaking on elections in Ukraine, he said "If elections are held under such terror as we see now, then we won't recognise them."
He added that pulling Russia's ambassador from Washington over the events in Ukraine would be a "last resort" and that he would "not like this to happen".
US Secretary of State Kerry pledged an aid package including US$1 billion in loan guarantees to Ukraine.
"We will do our best," he told the crowds, as he laid a bouquet of flowers at a memorial in Independence Square to those killed in the Ukrainian capital during last month's protests. "We hope Russia will respect the elections".
Russian soldiers earlier fired warning shots into the air as around 300 Ukrainian troops marched on the seized Belbek air base in Crimea.
The unarmed Ukrainian servicemen, waving flags, approached the base which has been held by Russian forces.
Around a dozen Russian soldiers warned them not to approach, before firing several shots into the air and reportedly saying they would shoot the Ukrainians if they continued to approach the base.
Meanwhile the European Union has given Russia until Thursday to pull its troops out of the Crimea region of Ukraine.
Russian troops fired warning shots at the Belbek air base in Crimea
The move has drawn international condemnation as the US President Barack Obama claims Russia has broken international law and says the country is on the "wrong side of history".
The US has temporarily suspended all military to military engagements with Russia.
It came as the Russian UN envoy told a stormy meeting of the United Nations Security Council that Ukraine's ousted leader Viktor Yanukovych had sent a letter to Russian President Vladimir Putin, requesting that he use the Russian military to restore law and order in Ukraine.
But US President Barack Obama warned Moscow it would find itself "on the wrong side of history" and that Russia's deployment of troops in Ukraine violates international law.
