Offer letter not sent for more than a week- HR making excuses

I have given an interview in a company. There were two rounds and I have cleared both. Consequently, HR asked me to send document on email. After I sent the documents I was told that the offer letter would be generated on the next day. This was 10 days ago but I still have not received the offer letter. I called the concerned HR official and they informed me that the offer letter could not be generated due to some technical difficulties.
Understandably, I am very worried. What should i do in the circumstances? Are the liable to generate and send me an offer letter?

First of all , please work on your written communication.
Don't worry They will get back to you. Please don't stop your searching, don't wait for them they will get around if they need you.

I smell something foul on deal because the plea of HR department is more smokey. You talk one more time with HR and ask the reason of delay and expected time. Do not be more aspirant over the matter on discussion you will understand well.
You are advised to look for alternative or consider anything that is on line.