The "preliminary round of talks" between the two parties was held at Chief Minister Oommen Chandy's official residence after Congress high command gave the green signal to explore the possibilities of co-operation with RSP, including an electoral tie-up, Congress sources said.
Congress is learnt to have sought an assurance from RSP that it would support UPA at the Centre in return for backing its candidate in Kollam seat.
Apart from Chandy, KPCC president V M Sudheeran and Home Minister Ramesh Chennithala and UDF convener P P Thankachan were present from Congress side. RSP was represented by its state secretary A A Azeez, V P Ramakrishna Pillai and Premachandran.
RSP leaders also made it clear there was no return to LDF and the party would back the UPA at the national level.
"This issue (the national stand) was also discussed. But at this juncture, we cannot divulge the details," Azeez said.
Earlier, Sudheeran said Congress viewed RSP's decision as a major development that shook the very foundations of LDF.
He said all UDF partners supported holding talks with RSP and before making subsequent decisions, they would again be consulted and the coalition would take a collective decision.
It also rejected the appeal of central leaders of CPI(M) and CPI to reconsider its decision in the larger interests of Left unity.
Kollam was once an RSP stronghold and the party had sound trade union base in the area which had several traditional industries like cashew processing.
RSP founder leader late N Sreekantan Nair had won from Kollam repeatedly. CPI(M) took over the constituency later and won in 2004.