Employer not providing relieving letter - what I can do to get the relieving letter?

I joined a private organization (OperativeMedia) in Bangalore in 2017. After working for nearly 1 and a half years I dropped my papers on 30th Oct 2018. I served the notice period of 2 months and left the organization on 27th Dec 2018. I wasn't handed over any relieving letter on the day I left the organization but the HR insisted that he will soon send the relieving letter in another 10 to 15 days.
Now more than a month has passed but he didn't send any relieving letter to me. I sent a mail reminding him of the letter and he said he'll be sending it in another couple of weeks. I think he is intentionally delaying and neither he provided me a resignation acceptance copy. Can anyone suggest, what I can do to get the relieving letter?

Hi
Keep contacting and communicating with the HR and after couple of emails mark cc to the management. When marking to the management make sure to acknowledge the email of the HR wherein he had promised that the relieving letter will be sent in 2 weeks time to bind him. You may use that email as your relieving letter just in case it is needed in the meantime. You may even indicate that you will be using his email ack as the relieving letter so long as you do not receive the official copy. Keep your letter of notice and other docs intact for any future use.
Thanks and Regards