Flight crew reported encountering an near midair collision event during climb.

Attributes
ACN	1758491
Time
Date	202008
Local Time Of Day	0001-0600
Place
Locale Reference	ONT.Airport
State Reference	CA
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	Widebody Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Person 2
Function	Pilot Flying First Officer
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Conflict NMAC
Miss Distance	Vertical 400

Narrative:

Climbing on the SID; departure told us to stop our climb at 6;000 ft. Instead of 7;000 due to traffic. We complied. Once we leveled off at 6;000 we saw 2 targets on TCAS at 12 o'clock; one at our altitude and one 500 ft. Above around 6 miles away. The target at our altitude disappeared off the TCAS screen but the one at 6;500 was still there and caused a TA (traffic advisory). The target then got closer but the TCAS showed it at 400 ft. Above and then finally gave us a preventative RA (resolution advisory) where the top arc of the TCAS was red. We finally saw the traffic which passed on our right side. Got the 'clear of conflict' announcement and reported the RA to ATC. VFR traffic transiting the airspace along with IFR traffic at altitudes [is] too close in my opinion. More vertical separation needed.

Original NASA ASRS Text

Title: Flight Crew reported encountering an NMAC event during climb.

Narrative: Climbing on the SID; Departure told us to stop our climb at 6;000 ft. instead of 7;000 due to traffic. We complied. Once we leveled off at 6;000 we saw 2 targets on TCAS at 12 o'clock; one at our altitude and one 500 ft. above around 6 miles away. The target at our altitude disappeared off the TCAS screen but the one at 6;500 was still there and caused a TA (Traffic Advisory). The target then got closer but the TCAS showed it at 400 ft. above and then finally gave us a Preventative RA (Resolution Advisory) where the top arc of the TCAS was red. We finally saw the traffic which passed on our right side. Got the 'Clear of conflict' announcement and reported the RA to ATC. VFR traffic transiting the airspace along with IFR traffic at altitudes [is] too close in my opinion. More vertical separation needed.

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.