Narrative:

We were operating aircraft X; on approach to the ILS xxr to ZZZ. We were approximately 2-3 miles outside zzzzz; established on the ILS xxr; cleared for the approach and fully configured for landing. IMC conditions existed with moderate mixed icing in the clouds. Everything was normal until we received the following TA in exactly this cadence: 'traffic; traffic! Climb; climb! Increase climb; increase climb!' I commanded go-around; and the first officer (first officer) immediately disconnected the autopilot; selected toga and initiated a go-around maneuver; while I assisted with gear and flap clean up. While we were engaged with this we entered VMC conditions and I observed out of my left peripheral view aircraft position lights to our left and slightly below us; closing on us with no relative movement. I recall pulling back on the yoke with the first officer and shouting 'climb; climb; climb!' as soon as we had done this we received a 'clear of conflict' message. This all took place in just a few seconds. I announced our go-around to ZZZ tower; and we were told to track the 16R localizer course and climb to 9;000. When asked the reason for the go-around we told them we were responding to an RA. We were then handed off to ZZZ approach; who advised us that there was another aircraft also on a go-around climbing to 11;000 feet. I'm not sure if this was the aircraft that caused our loss of separation; but I suspect so. We were then vectored for a subsequent approach to the ILS xxl which was accomplished uneventfully.without hearing radio transmissions from the other tower frequency I can't say for sure why the other aircraft was suddenly encroaching on our flight path; but I can say that the TCAS went from nothing to a red target to our immediate left with RA commands almost instantly. I don't know how close we came to the other aircraft; but in the brief time that we were VMC it seemed to me that we were very close.this is only conjecture; but I believe that an aircraft on the parallel approach to xxl executed a go-around; but failed to fly the localizer course and began turning into us; thus creating the loss of separation and RA event. For our part; we followed sops; as well as ATC directions. First officer did an exceptional job of flying the aircraft first and getting us out of the path of the other aircraft. He also stated that when we received the RA; his vsi was completely red; with no safe 'green band' to even fly into.

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Original NASA ASRS Text

Title: B737 flight crew reported that an unknown aircraft created a traffic conflict on final; and resulted in an RA and a go-around.

Narrative: We were operating Aircraft X; on approach to the ILS XXR to ZZZ. We were approximately 2-3 miles outside ZZZZZ; established on the ILS XXR; cleared for the approach and fully configured for landing. IMC conditions existed with moderate mixed icing in the clouds. Everything was normal until we received the following TA in exactly this cadence: 'traffic; traffic! climb; climb! increase climb; increase climb!' I commanded Go-Around; and the FO (First Officer) immediately disconnected the autopilot; selected TOGA and initiated a go-around maneuver; while I assisted with gear and flap clean up. While we were engaged with this we entered VMC conditions and I observed out of my left peripheral view aircraft position lights to our left and slightly below us; closing on us with no relative movement. I recall pulling back on the yoke with the FO and shouting 'climb; climb; climb!' As soon as we had done this we received a 'clear of conflict' message. This all took place in just a few seconds. I announced our go-around to ZZZ Tower; and we were told to track the 16R localizer course and climb to 9;000. When asked the reason for the go-around we told them we were responding to an RA. We were then handed off to ZZZ Approach; who advised us that there was another aircraft also on a go-around climbing to 11;000 feet. I'm not sure if this was the aircraft that caused our loss of separation; but I suspect so. We were then vectored for a subsequent approach to the ILS XXL which was accomplished uneventfully.Without hearing radio transmissions from the other Tower frequency I can't say for sure why the other aircraft was suddenly encroaching on our flight path; but I can say that the TCAS went from nothing to a red target to our immediate left with RA commands almost instantly. I don't know how close we came to the other aircraft; but in the brief time that we were VMC it seemed to me that we were very close.This is only conjecture; but I believe that an aircraft on the parallel approach to XXL executed a go-around; but failed to fly the localizer course and began turning into us; thus creating the loss of separation and RA event. For our part; we followed SOPs; as well as ATC directions. FO did an exceptional job of flying the aircraft first and getting us out of the path of the other aircraft. He also stated that when we received the RA; his VSI was completely red; with no safe 'green band' to even fly into.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.