NEW DELHI: With the Left issuing an ultimatum on the Indo-US nuclear deal, a meeting of the UPA leaders is being held here on Sunday to decide ways and means to tackle the situation which has caused a crisis for the three-year-old government.

The meeting is part of the consultations the top Congress leadership has initiated with allies to find a way out of the situation at a time when the key outside supporters are saying that the minimum they expected was non-operationalisation of the deal.

Soon after the Left ultimatum, top brass of Congress including Prime Minister Manmohan Singh and party chief Sonia Gandhi had held 90 minute deliberations at the Congress core group meeting on Saturday.

After the meet, some senior leaders including External Affairs Minister Pranab Mukherjee and Defence Minister A K Antony had held consulations with RJD supremo Lalu Prasad while Gandhi had telephonic consultations with some of the allies.

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.

The CPI(M) Politburo on Saturday passed a resolution describing the deal as "unacceptable" and demanding that the government should not proceed further on it by commencing negotiations with the UN nuclear watchdog IAEA for safeguards which would bind India in perpetuity.