Flight Crew Air Transport Pilot (ATP)
Conflict Airborne Conflict
Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory
Environment - Non Weather Related
While descending on the Milton arrival into LGA; leveling at 10;000 ft; we received an instruction to fly heading 100 from ATC due to traffic as we simultaneously received a TCAS RA. The RA instructed us to descend; this command was followed and ATC was notified. Following this command resulted in us being at approximately 9;300ft at 310 kts. After we were clear of conflict ATC advised us to maintain 9;000 ft and the aircraft was immediately slowed to 240 kts. Flight Crew was uncertain if this RA conflict resulted in another RA from an aircraft lower; or if it was just an advisory.Late instructions from ATC with regards to VFR traffic avoidance. TCAS RA required an immediate descent below 10;000 MSL without opportunity to slow the aircraft below 250 kts. Recommend better coordination between sector controllers and VFR aircraft operating in the Class B airspace.
EMB-145 First Officer reported exceeding 250 knots below 10;000 FT in response to an RA on arrival into LGA.