SLAVIANSK, Ukraine — Russian mercenaries with experience fighting in Syria and Libya have covertly trickled into two rebel territories in eastern Ukraine, helping to lay the groundwork for war, according to two senior European security officials.
The mercenaries, who so far number about 300, are with the Russian paramilitary group Wagner and arrived in the separatist enclaves of Donetsk and Luhansk wearing civilian clothes, according to the European officials. Western intelligence services have tracked them leaving Libya and Syria and arriving in Russian-controlled Crimea, from where they have filtered into the rebel territories, one of the officials said.
Their numbers are tiny compared to the estimated 190,000 troops that President Vladimir V. Putin of Russia has placed around Ukraine’s border as he threatens to wage what many fear could be the bloodiest conflict in Europe since World War II.
But the presence of Wagner fighters is another ominous sign of the approach of war, and raises the possibility that Mr. Putin may follow a playbook used in 2014, when the Kremlin deployed Russian mercenaries, mostly veterans of the Russian military, to augment the forces of rebel fighters in eastern Ukraine.