VIENNA, Dec. 18 - European security watchdog the OSCE prolonged the mandate of its observer mission at two Russian checkpoints on the border with Ukraine by three months on Thursday, the longest extension so far, Reuters reported.

Ukraine and the West have accused Russia of sending weapons and soldiers to help pro-Russian separatists in the country's east during the conflict that began after the overthrow of Ukraine's Moscow-backed leader in February. Russia denies it.