I think you should let the other company know you have an offer on hand. Do it tactfully and they will likely to expedite for you if they are keen to hire you. Also, they will bear that in mind and give you what they can offer.

Negotiation of salary is always done before acceptance. Once accepted, its very tough to negotiate unless you want to play the card "I would very much want to join you, but another company is offering me X amount more".

To play this card, you need to really have another offer from B in case A really turns you down.

Good luck and other forumers are right, you are free to accept first, work and search for a better job. There is absolutely no obligation to stay in a job, even after 1 week, if another company can offer better pay and prospect.

Remember this is a free market economy - do what is best for yourself. good luck!