KYIV, Dec 12 (Reuters) - Ukrainian forces said they had retaken parts of the northeastern town of Kupiansk and had encircled Russian troops there as President Volodymyr Zelenskiy visited the area and praised the operation, saying it strengthened Ukraine diplomatically. With U.S.-backed peace efforts underway, Moscow has said it is advancing on all fronts and that it has seized Kupiansk and the strategic city of Pokrovsk in the east. Kyiv has denied this, saying that the fighting is continuing.