Stymying the Manmohan Singh government?s efforts to push ahead with the Indo-US civilian nuclear cooperation agreement, Left parties on Friday refused to back the deal in its current form and asked the government not to proceed with operationalising it.

The four-party alliance, which extends critical outside support to the UPA government, also decided to press for a constitutional amendment for bringing international treaties and certain bilateral agreements for approval in Parliament. They also called for a review of strategic aspects of the Indo-US relations in Parliament.

The Left stand, articulated in a five-page statement following discussions within the Left parties on Tuesday evening, has effectively put the deal under a cloud even as government managers said it would talk to them.

The Left decision to oppose the deal now puts the ball squarely in the Centre?s court, which will now have to take a call on whether to proceed ahead with negotiations with the IAEA as well as the Nuclear Suppliers Group (NSG).

The government was scheduled to make a suo motu statement on the issue in Parliament next Monday, but it may now have to alter its plans and attempt a consensual approach within the ruling alliance.

Given that the UNPA and the BJP have already raised objections to the deal, the opposition by the Left now virtually isolates the Congress on the issue in Parliament.

To a volley of questions on how the Left planned to ?block? the government from going ahead with the deal, CPI(M) general secretary Prakash Karat said, ?We have not discussed our Parliament strategy. We will discuss it further separately.? Asked whether their unhappiness over the deal would lead to withdrawal of support, he remained non-committal saying it was a separate issue, which has not been discussed.

The Left maintained that the 123 Agreement should not be viewed separately but in conjunction with the Hyde Act.

They contended that provisions of the Hyde Act were far wider than the 123 Agreement and could be used the terminate the deal not only in the eventuality of a nuclear test but also for India not conforming to US foreign policy.

?The termination clause is wide ranging and does not limit itself to only violation of the agreement as a basis for cessation or termination of the contract,? it said.

In the eventuality of termination, therefore, India would be back to complete nuclear isolation, while accepting IAEA safeguards in perpetuity, the Left statement said.

Maintaining that the 123 Agreement fell short of the assurances made by Prime Minister Manmohan Singh to Parliament, the Left said, ?While the Indian commitments are binding and in perpetuity, some of the commitments that the US has made are either ambiguous or are ones that can be terminated at a future date.?

For instance, it said that the Prime Minister had assured that the agreement would cover the entire nuclear fuel cycle. However, the text of the agreement denied full cooperation or access in any form to fuel enrichment, reprocessing and heavy water production technologies, the Left said.