CRJ200 flight crew reports strong fuel odors during approach after starting the APU. APU is secured and fumes go away.

Narrative

Inbound to ZZZ at 10000 ft and 250 kts; we attempted to start the APU and it did not light off. We discontinued the start and turned off the power/fuel. Then after at least 2 minutes we tried to start the APU a second time at about 6;000 ft and it started normally. After 1 minute and before we transferred the bleeds we encountered a very strong odor of jet fuel with no EICAS cautions or warnings. We then donned the oxygen masks and established communication. We continued the approach into ZZZ since it was the nearest suitable airport and declared an emergency with the Crash Fire Rescue trucks standing by. We contacted the flight attendant and asked her if she noticed the jet fuel fumes and she answered yes. We told her to say there is an emergency and will be landing soon. We landed without incident and stopped on the taxiway to let crash fire rescue check out the aircraft for fire or excessive fuel leakage. There was none and we taxied to the gate with Crash Fire Rescue following us. We contacted operations; Dispatch and maintenance about the event.