Termination of service without giving Appointment Letter

Dear friends, I am working in a pharmaceutical company as the Zonal Head. I joined the organisation on 18th April 2017. Till date I have not received my appointment letter. I have received only the offer letter which I had accepted. However I have been receiving my salary and expenses reimbursements every month and my salary slips too. On 12th Aug the HR manager asked me to submit my performance report in a set format which I did the same day. On 21st August he replied me that my performance is not up to the level that been expected. Also stated that I am advised to improve my performance by end of this month and if the performance is not at satisfactory level the company may go for liable action. Now my queries are like this 1. Since I have not received my Appointment Letter and signed the same I am not aware of the expected parameters of performance set by the company. Can my services be terminated ? 2. Can my services be terminated just on the ground of non performance that too according to the company whereas I have not agreed to any terms and conditions. 3. If I will be asked to resign what should be my course of action ? 4. If my service gets terminated can I file a case in Labor Court ? If yes, then what should be the grounds ? 5. Can I negotiate for six months salary as a compensation for leaving the job Thanks