Zurich: US Secretary of State John Kerry on Saturday called on Russia to "immediately" stop backing Ukrainian separatists after at least 30 people were killed in the strategic port of Mariupol in a major escalation of the fighting in eastern Ukraine.
"We call on Russia to end its support for separatists immediately, close the international border with Ukraine, and withdraw all weapons, fighters and financial backing," Kerry said during a visit to Zurich.
"Otherwise, US and international pressure on Russia and its proxies will only increase," he warned.
The top US diplomat said he condemned in the strongest terms the "horrific" assault on Mariupol.
The government-held city was hit by heavy rocket fire on the day that pro-Kremlin rebels announced an offensive against Mariupol.
Kerry said the assault was "in blatant violation of the Minsk (peace) agreements they signed".
The rebels` new offensive "has been aided and abetted by Russia`s irresponsible and dangerous decision to resupply them in recent weeks with hundreds of new pieces of advanced weaponry, including rocket systems, heavy artillery, tanks, armoured vehicles, in addition to continuing operational command and control," he added.
Both the United States and the European Union have slapped sanctions on Russia over its role in the Ukraine crisis. Russia denies arming or fighting alongside the rebels.