LONDON, May 19 (Reuters) - Russian military forces have been enhancing defensive positions in and around the Zaporizhzhia nuclear power plant in southern Ukraine in recent weeks, four witnesses said, ahead of an expected counteroffensive in the region. But there is concern in the international community that the six-reactor nuclear plant, Europe's largest, could be caught up in fighting, particularly as military analysts expect Ukraine to try to push Russian forces back in the Zaporizhzhia region. The plant's reactors were all shut down by last September and the number of workers has fallen from around 11,000 Ukrainian staff before the war to around 6,000, Energoatom says