Biden and Putin exchange warnings during phone call amid rising Ukraine tensions

• Talks represent pair's second conversation this month
• Russia massing tens of thousands of troops near border

Joe Biden and Russian counterpart Vladimir Putin have exchanged warnings over the crisis in Ukraine during a 50-minute phone call that did little to lower the political temperature, according to their governments.

Russia has alarmed the US and its allies by massing tens of thousands of troops near its border with Ukraine over the past two months. This follows its seizure of Ukraine's Crimea peninsula in 2014 and its backing of separatists in eastern Ukraine.