Large-scale intervention in eastern Ukraine by regular Russian troops began last August, reaching a peak of 10,000 in December, and Moscow has been struggling to maintain operations on such a scale and intensity, according to a report.The report, by the Royal United Services Institute (Rusi), claims small teams of reconnaissance and Spetsnaz special operations units crossed the border earlier, in mid-July after Ukrainian government forces had won a series of battles and had pushed pro-Moscow separatists out of territory they had previously occupied.
The Rusi report also confirms the findings of a February investigation, based on analysis of satellite imagery by the Bellingcat group of investigative journalists, that Russian artillery shelled Ukrainian positions from inside Russian territory.
Vladimir Putin, the Russian president, has admitted planning the invasion and annexation of Crimea last March, but his government continues to deny any direct involvement in the gruelling conflict in eastern Ukraine. However, the Rusi report says that a total of 42,000 Russian troops from 117 combat and combat-support units have been involved, either being rotated in and out of the front lines in Ukraine or pouring artillery fire from inside Russia.