Sonia is understood to have discussed the future strategy of the UPA in the wake of the Left parties, which are providing crucial outside support to the government, serving an ultimatum to the government on the deal.
Earlier, External Affairs Minister Pranab Mukherjee met Rashtriya Janata Dal chief Lalu Prasad and his party colleague P C Gupta and is understood to have deliberated upon the future course of action.
Prime Minister Manmohan Singh and Gandhi are expected to hold confabulations with leaders of UPA constituents at a meeting in New Delhi on Sunday.
A meeting of the Congress Core Group, chaired by Gandhi, "noted" the statements of the Left and deliberated on the pros and cons of the matter.
The 90-minute meeting of the core group was attended by Singh and senior ministers Pranab Mukherjee, A K Antony.
A senior Congress leader said consultations were on and "there is nothing to worry about."
The four Left parties -- CPI-M, CPI, RSP and Forward Bloc -- who are extending crucial outside support to the UPA coalition have virtually put the government on notice with the demand that further action on implementation of the 123 agreement, including talks with IAEA and NSG, should not be taken up.