Software Company Denying To Relieve Even After The Notice Period Served.

Hi, I m working as a software eng in an office in noida. I served my notice period but the company is forcing me to complete a task which is of not just one or two days although I tried it and extended few days further. They are saying to finish that task then only they will give my experience letter and my two months notice period salary. I have joining in next company. I am stuck please help.