Putin says only local forces of self-defence in Crimean standoff.
United States President Barack Obama says Russia is not "fooling anybody" over the crisis in Ukraine after Vladimir Putin denied Russian forces were operating on the flashpoint Black Sea peninsula of Crimea.
In a show of support for the new interim leaders in Kiev, visiting US Secretary of State John Kerry condemned Russia's "act of aggression" and accused Moscow of "working hard to create a pretext for being able to invade further".
Pro-Kremlin forces are in de facto control of the strategic, majority-Russian Crimean peninsula where Ukrainian troops remain blocked inside their barracks in the most serious standoff between the West and Russia since the end of the Cold War.
Amid the tensions, Russia carried out a successful test launch of an "advanced" intercontinental ballistic missile, state news agencies reported. The US was informed of the test this week, a US defence official said.
World financial markets meanwhile rebounded yesterday as traders interpreted comments by Putin that force was "a last resort" as a sign that the situation in the nation of 46 million people was easing.
Ukraine's interim Prime Minister, Arseniy Yatsenyuk, also announced his Government had made the first "timid" contacts with Russian leaders aimed at resolving the crisis.
But the rhetoric from the US and Russia remained tough.
Obama said the European Union and allies such as Canada and Japan all believed Russia had violated international law by mobilising troops after the February 22 ouster of pro-Kremlin President Viktor Yanukovych.
"President Putin seems to have a different set of lawyers, maybe a different set of interpretations. But I do not think that is fooling anybody," Obama said.
Putin, breaking more than a week of silence with a press conference in Moscow, said his country reserved the right to use "all available means" to protect Russians in Ukraine. "This is a last resort," he added.
Responding to claims by Ukraine's new authorities that thousands of Russian troops had poured into Crimea in recent days, Putin said only "local forces of self-defence" were surrounding Ukrainian military bases there.
Asked if Russian forces took part in operations in Crimea, he said, "No, they did not participate", adding: "There are lots of uniforms that look similar."
When told of Putin's remarks by a reporter in Kiev, Kerry responded: "He really denied there were Russian forces in Crimea?" and shook his head, bewildered.
The top US diplomat also laid flowers and lit a candle at the scene of memorials to the nearly 100 people who died in clashes on Kiev's Independence Square last month.
The square was the focal point of three months of protests sparked by Yanukovych's decision to reject a key European Union deal in favour of closer ties with Moscow.
In a further public boost to Ukraine's new Western-backed authorities, the US confirmed US$1 billion ($1.2 billion) in loan guarantees to help shore up Ukraine's debt-laden economy.
The European Commission also offered an aid package reportedly worth more than 1 billion ($1.6 billion), as cash-strapped Ukraine says it needs 25 billion over two years.
There was no immediate sign of calm returning to Crimea, which has housed the Russian Black Sea Fleet since the 18th century.
In one incident, Russian forces fired warning shots in the air as unarmed Ukrainian soldiers approached them at a base near Sevastopol in what appeared to be the first shots fired since the Crimean crisis erupted.
In Sevastopol, a bastion of pro-Kremlin sentiment, the headquarters of the Ukrainian navy in Crimea was surrounded by about 100 pro-Russian activists who formed a human chain as Russian forces looked on.
Ukrainian defence officials said Russian vessels were also blocking Ukrainian warships from trying to leave the port.
In a sign of profound Western frustrations with Russia, Kerry warned that unless Russia acts to reduce tensions, it will face further international action after the US already suspended military co-operation.
"If Russia does not choose to de-escalate," he said, "our partners will have absolutely no choice but to join us to continue to expand the kind of steps we have taken in recent days in order to isolate Russia politically, diplomatically and economically."
A US official travelling with Kerry said Russia was "likely" to face US moves to introduce sanctions as early as this week. The EU has also threatened "targeted measures".
In Paris, French President Francois Hollande warned that Russia was "running the risk of a dangerous escalation".
Along with Europe, France would "apply all necessary pressure including sanctions to find the path of dialogue and a political issue to this crisis", Hollande said.
But Putin warned that any sanctions would lead to "mutual losses".
Kerry was to come face-to-face with his Russian counterpart, Sergei Lavrov, in Paris overnight, their first encounter since the start of the Crimea turmoil.
The two were scheduled to take part in a conference on Lebanon, with the Foreign Ministers of Britain, France and Germany also attending. The events in Ukraine are expected to overshadow the talks.
Also on today, EU foreign policy chief Catherine Ashton is to visit Kiev before an EU emergency summit on Ukraine tonight and tomorrow.