Whether company can cancel an extend employment offer letter?

I have received the extend offer letter with salary break up last 1 month back. But till date they did not issued offer letter. Kindly suggest can they cancel this extend offer. I called HR and they are telling that offer letter is pending for approval. As my boss of current employer came to know that I may resign so he is also now pressuring me. I am not understanding whether to resign from current employment.

There is always a chance that the offer letter may get cancelled.
Job searches are always to be done discreetly.
Your present employer has become doubtful of your interest to continue which may impact upon you.
You need to clarify from new employer HR and sense from what is being said or unsaid and take a decision.
Do not let things hang in between.
Remaining employed is always important.

Can I directly call new HR that to issue the offer letter on urgent basis or to reject the extend offer

There is nothing wrong or inappropriate in contacting the prospective employer and explain your situation and depending on the response you can decide the situation.

My advice is not to resign. You better devout fully in present job. Do not consider the call of the new employer as they do not have time schedule, the rest is up to you.