POTENTIAL CONFLICT DEVELOPS DURING ALTDEV ALT OVERSHOT IN DSCNT. ACR X FREIGHTER WITH OPPOSITE DIRECTION TFC OF A B747 700 FT BELOW. OCCURRED IN FOREIGN AIRSPACE.

Narrative

ON DSCNT FROM FL330 TO FL290 I HAD CALLED PASSING FL300 FOR FL290 AND TURNED MY ATTN TO NAV DUTIES. I LOOKED BACK AT THE INST PANEL SHORTLY AND SAW US PASSING FL290. I CALLED ATTN TO THIS AND GRABBED THE YOKE TO STOP DSCNT AND CLB BACK TO FL290. WE HAD PROBABLY GONE DOWN AS FAR AS FL287. ONCE BACK LEVEL AT FL290 ATC ADVISED US TO MAINTAIN FL290 AS WE HAD OPPOSING TFC. THE TFC WAS ANOTHER ACR Y B747 AND WE DID HAVE A VISUAL ON THEM. JUST AFTER ATC REMINDED US TO MAINTAIN FL290 I LOOKED OUT AND SAW THE B747 IN A SLIGHT R TURN. THEY ASK ATC FOR OUR FLT NUMBER AND ADVISED THEY WERE MAKING A TURN AS THEIR TCASII HAD ALERTED MERGING TFC WITH LESS THAN 1000 FT SEPARATION. I ADVISED THEM WE WERE LEVEL AT FL290 AND THEY ACKNOWLEDGED. ATC DID NOT RESPOND. THIS INCIDENT WAS CAUSED A MOMENTARY DISTR OF ATTN TO OTHER DUTIES IN THE COCKPIT ON ARR; AND ABETTED BY FAILURE OF THE ALT ALERT SYS TO SOUND OR LIGHT. SUPPLEMENTAL INFO FROM ACN 344956: AT THIS TIME THE AUTOPLT WAS DISENGAGED AND MANUALLY CLBED BACK TO FL290. THE OTHER ACFT RPTED THAT THEIR TCASII GAVE THEM A WARNING OF CONFLICTING TFC OF LESS THAN 1000 FT. WE HAD THE TFC IN SIGHT AT ALL TIMES EVEN BEFORE GOING BELOW FL290 BY 200 FT. I BELIEVE HAD THE ALT ALERT SYS BE WORKING IT WOULD HAVE HELPED TO AVOID THE SIT IN A VERY BUSY COCKPIT ENVIRONMENT.