An A319 flight crew intercepting an ILS approach climbed in response to a TCAS RA triggered by a helicopter which ATC had advised; level 500 FT below their altitude.

Narrative

As we turned final the Approach Controller cleared us to descend to 3;500 FT. After we read back the altitude and started our 500 FT descent we were advised of VFR helicopter traffic under BALAS at 3;000 FT and that this traffic had us in sight. We responded that we were looking for the traffic. While level at 3;500 FT on the localizer; we received a TA and saw the traffic on TCAS 500 FT below us. The target appeared to climb until it was directly below us showing less than 400 FT; at this point we received a RA of 'climb; climb' and a vertical speed of at least 600 FT/min was necessary. We informed ATC of the RA as we were climbing. Our altitude deviation was 400-500 FT. We never had visual contact with the helicopter. As we were still recovering from the RA the Controller attempted to clear us for the visual approach; we asked ATC to standby as we were still leveling off and clearing our altitude alarm. ATC then cleared us for the visual for 22L again. We proceeded with the approach and were stabilized by 1;000 FT AGL.