E145 CREW HAD LESS THAN LEGAL SEPARATION IN ZJX CLASS A AIRSPACE.

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OPERATING AS ACR FLT ABC MYR-CLE. ZJX CLRED US TO FL290 IN CLB FROM MYR. I FIRST NOTICED CONFLICTING TFC ON TCASII WHEN HE WAS AT 15 NM AT 10 O'CLOCK POS. HE APPEARED LEVEL AT FL290; AND WAS CLOSING WITH US. AT APPROX 5 NM WE SIGHTED HIM VISUALLY. AS WE CLBED THROUGH FL285; ATC CALLED US TO CONFIRM WE WERE LEVEL AT FL280. PNF REPLIED WE HAD BEEN CLRED TO FL290 AND THAT WE HAD THE TFC VISUALLY. THE TFC PASSED APPROX 3 NM BEHIND US AND 500 FT ABOVE. UPON REVIEW OF THE SIT WITH THE CAPT; WE BOTH AGREED THAT WE HAD BEEN CLRED TO FL290; THAT I HAD SELECTED FL290 IN ALT PRESELECT BOX; AND THAT HE HAD CONFIRMED IT. A DIFFERENT ATC VOICE THEN ADVISED US TO CONTACT ZTL. WE DID RECEIVE A TA ON THE TFC; BUT NOT AN RA. I FEEL THAT EVEN IF THE ATC CTLR BELIEVED THAT WE WERE CLBING TO FL280; HE SHOULD HAVE ADVISED BOTH ACFT OF THE OTHER'S PRESENCE. THIS WOULD CERTAINLY HAVE AVOIDED THE LOSS OF SEPARATION. ADDITIONALLY; I FEEL THAT ATC SHOULD HAVE NOTIFIED US ASAP ONCE CTLR ERROR WAS DISCOVERED IN ORDER TO BETTER ALLOW US TO CONCENTRATE ON OUR DUTIES RATHER THAN WONDER WHETHER WE HAD BEEN AT FAULT.