I just don't want to lose the offer...

I would need far more details to offer any ideas. What more can you tell me about this situation?

The basic answer is yes, at this point you have a offer, you have three real choices, accept, reject or counter.

When you counter you are not accepting, so they are free to withdraw the offer. In fact most companies assume you have rejected the first offer and now counter, so they may merely go and hire someone else.

It is a risk, often they will counter back at a slightly lower amount than your request or say that sorry their first offer was the highest.

They may also assume you would not be happy with the first offer and just go hire someone else that would be.

It is entirely up to you and there is no always on what a company may do.