WASHINGTON : The United States called on Russia Sunday to "immediately" observe the ceasefire in Ukraine, accusing combined Russian and separatist forces of targeting international monitors.

"We call on Russia and the separatist forces it backs to immediately observe the ceasefire, withdraw all heavy weapons, and allow full and unfettered access to the OSCE monitors," the State Department said. State Department spokesman Mark Toner said the United States was closely monitoring growing violence in eastern Ukraine and the failure of the combined Russian and separatist forces to abide by a ceasefire agreed to two years ago in Minsk.

"We condemn Friday's targeting of OSCE Special Monitoring Mission (SMM) monitors and the seizure of a SMM unmanned aerial vehicle by combined-Russian separatist forces," Toner said, referring to the civilian observers with the Organization for Security and Cooperation in Europe.

"It is imperative that these forces halt their attacks on civilian infrastructure, including the Donetsk water filtration station."

The US statement comes amid an upsurge of violence with Ukraine reporting on Saturday that 16 of its soldiers were wounded over the previous 24 hours.

The latest casualties came despite a truce agreed on Monday by Russia and Ukraine. The nearly three-year-long conflict has claimed more than 10,000 lives.