Defending the secrecy clause, over which a political battle is raging, Attorney General KK Venugopal said, "These matters are for the experts to deal with... we have been saying that even Parliament has not been told about the complete cost of jets".

The information, he added, can't be disclosed even under the Right to Information Act. "The price (revealed in parliament) was not of loaded aircraft. Secrecy is on weapon and avionics. The Air Force feels if it is disclosed, our adversaries will take benefit," Mr Venugopal told the bench, which is being led by Chief Justice of India Ranjan Gogoi.

The court questioned the government's stance that it had no role in the offset clause, the heart of the opposition's corruption allegations. The offset contract - which the government said runs concurrently with the main contract -- cannot be separated, the court said. "It may not be in the country's interest if the offset contract is executed later because that may lead to delay in production by the offset partner," the judges said.

Lawyer-activist Prashant Bhushan, who is one of the petitioners, questioned how revealing the pricing of the aircraft can compromise national security. "It was disclosed in Parliament twice... The government is now saying what was stated in Parliament is the basic price. The government is hiding behind the secrecy agreement," he said.

The court sought the help of IAF, saying it wanted to "hear from an Air force officer and not the official of the Defence Ministry on the issue." Deputy Chief of Air staff, Air Marshal V R Chaudhari, and two other officers told the court that no new aircraft has been inducted in the Air Force since 1985.

BJP veteran Arun Shourie, a minister in Atal Bihari Vajpayee's government and one of the petitioners in the case, told the court that Manohar Parrikar, who handled the defence portfolio before Nirmala Sitharaman, was initially not aware of the amended deal between India and Dassault. "In an interview Manohar Parrikar said '90,000 crore for 126 planes'," Mr Shourie said.

The government's justification for the new deal -- with a change in offset guidelines -- which was made effective from a retrospective date, was India-specific add-ons and secrecy agreement, Mr Shourie said. "Parrikar had said the new deal is because of political developments between the two counties," he added.

On Monday, the Centre had submitted the pricing details of the Rafale jets to the court in a sealed cover. It had also filed a 14-page document titled "Details of the steps in the decision-making process leading to the award of 36 Rafale fighter aircraft order" to the petitioners.

The petitions seeking a court-monitored investigation into the Rafale deal were first filed by advocates Manohar Lal Sharma and Vineet Dhanda. Later, Aam Aadmi Party lawmaker Sanjay Singh filed another one. Former Union ministers Yashwant Sinha, Arun Shourie and activist lawyer Prashant Bhushan also filed a joint petition in the top court.