Poachers killed 57 rhinos in South Africa during January, as illegal killings of the critically endangered species remains rife and efforts to combat the occurrence are meeting stiff armed resistance from criminal gangs. Several armed poachers were killed in firefights with rangers at the massive Kruger National Park in the country's north. Authorities arrested 18 people this month in connection with poaching, dpa reported. "Our operations are more militaristic," said David Mabunda, chief of South African National Parks authority. He noted that "aggressive incursions from Mozambique" were hindering the crackdown on poaching. Rangers and police recovered high caliber weapons, ammunition and silencers when making arrests. The rhinos are most often killed for their horns, which are then smuggled to Asian nations, where some believe they have medicinal value, though the healing potential is not scientifically proven. Last year, some 660 rhinos were killed in South Africa. The number stood at 440 killed in 2011. The country is home to the world's largest rhino population, estimated at about 20,000, though the large upsurge in poaching is threatening the ancient animal's existence.