Company Increasing Notice Period Without My Consent

Hi, I need some legal advice regarding my resignation. I submitted my resignation on 18th June 2025, clearly stating that I would serve a 15-day notice period, as mentioned in my signed offer letter. My proposed last working day is 2nd July 2025. However, HR responded saying I must serve a 60-day notice period, which I never agreed to and is not mentioned in my contract. I’ve politely denied serving 60 days and asked them to acknowledge my resignation based on the original agreement. They haven't formally accepted it yet, and I’m unsure of the next steps. Can an employer legally force a longer notice period than what's in the contract? What legal action can I take if they delay or refuse to relieve me? Any help or guidance would be appreciated. Thanks!