Union Home Minister Amit Shah has termed illegal infiltration and unnatural demographic changes a serious national challenge, saying the Centre is preparing a major action plan against them.Shah's warning comes amid a strict anti-infiltration drive launched by West Bengal's new BJP government. Reports say suspected illegal migrants have gathered near the Bengal border to return to Bangladesh as the drive intensified.The Home Minister did not release operational details of the plan. The remarks signal a harder line on border security and identity verification in border states.Opposition parties have in the past accused the government of using the infiltration issue for political ends, while the BJP argues demographic changes in border districts threaten national security and local resources.The drive has already triggered panic in border areas, with reports of migrants attempting to cross back. Civil society groups have urged the government to ensure that genuine citizens are not harassed in the verification process.