NMAC BTWN AN LGT DSNDING FOR A VISUAL APCH AND A LIGHT TWIN CLBING 15 MI FROM THE ARPT.

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ON DSCNT INTO LAS 8000 FT; 250 KTS; WE RPTED WHEN ASKED THAT WE HAD THE ARPT IN SIGHT. WE WERE THEN CLRED FOR THE VISUAL APCH FOR RWY 25L AT LAS. THE FO WAS THE PF. THE ACFT WAS ON AUTOPLT IN LNAV ON CTRLINE FIX FOR RWY 25L AND FLT LEVEL CHANGE. I NOTICED A TARGET ON TCASII ABOUT 1-2 O'CLOCK WITH NO ALT READOUT. ATC ALSO RPTED A TARGET AT 12-1 O'CLOCK WITH NO ALT RPT. AT THIS POINT I ASKED THE FO TO LEVEL OFF; WHICH HE DID IMMEDIATELY BY HITTING ALT HOLD. I WAS ASSUMING THE TARGET WAS BELOW US SINCE THERE IS A SMALL ARPT IN THE VICINITY; AND WE WERE JUST NEAR THE EDGE OF CLASS B AIRSPACE. I WOULD SAY ABOUT 3 SECONDS AFTER WE STOPPED THE DSCNT; THE RA FUNCTION OF THE TCASII COMMANDED A CLB. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND STARTED TO CLB. WHEN THE TCASII RPTED CLR OF TFC; THE FO RESUMED CTL. I LOOKED OUT THE L WINDOW AND SAW A LIGHT TWIN PASS FROM R TO L BELOW US. I RPTED THE RA TO ATC. AT THIS POINT WE WERE HIGH ON THE GLIDE PATH AND NEEDED THE GEAR AND SPD BRAKE TO GET BACK ON GLIDE PATH. A NORMAL LNDG WAS MADE. I THINK OUR SPD 240-250 KTS AND DSNDING WAS A FACTOR IN THE TCASII RA WARNING.