ACR IN LEVEL FLT RECEIVED A TCASII RA AND CLBED INTO TFC ABOVE RESULTING IN LTSS.

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CRUISING IN IMC AT FL280; 25 NM E OF FOD VOR ON IFR FLT PLAN TO PDX. WITH NO ADVISORY FROM ATC AND NO TFC NOTED ON TCASII WE SUDDENLY RECEIVED AN RA FROM THE TCASII ADVISING 'CLB.' BOTH CAPT AND MYSELF IMMEDIATELY LOOKED AT THE RADAR SCOPE AND NOTED A RED SQUARE IMPOSED ON OUR POS. THE CAPT IMMEDIATELY DISCONNECTED THE AUTOPLT AND BEGAN AN AGGRESSIVE CLB AS I LOOKED AT THE VSI AND STATED TO THE CAPT THAT THE CLB SEGMENT OF THE VSI WAS SOLID RED -- NO GREEN. THE CAPT STOPPED THE CLB AT ABOUT FL287 AS I CALLED ATC TO FIND OUT WHAT AND WHERE OUR CONFLICT WAS. THE AURAL WARNING GAVE A REPEATED 'MONITOR VERT SPD' ALERT AS WE WERE ATTEMPTING TO FIND A GREEN BAND TO FLY TOWARD. ATC ADVISED US AFTER A NOTICEABLE PAUSE THAT OUR TFC WAS AN MLT Y AT OUR 6 O'CLOCK POS; 2 MI AT FL290. THERE WAS NO WARNING AND NO ALERT OF ANY TYPE NOTED UNTIL THE ALERT OCCURRED SEEMINGLY AT OUR EXACT POS. RADAR SCOPE WAS ON AT THE 50 MI POS AT THE TIME OF CONFLICT. I BELIEVE THE CREW ACTED AND RESPONDED IN AN APPROPRIATE MANNER WITH THE CAPT TAKING COMMAND OF THE ACFT AND THE COPLT ADVISING AND GATHERING INFO. WHAT IS DISTURBING IS THAT THE ACTION COMMANDED BY THE AURAL WARNING WAS NOT SUPPORTED BY INFO ON THE VSI AND IN FACT LED US CLOSER TO A SERIOUS CONFLICT.