NEW DELHI  -Indian Prime Minister Manmohan Singh pledged on Thursday to crack down on corruption in the defence sector after a string of recent graft scandals left his government facing renewed calls to step down.

The premier said he was committed to making the process of buying arms and other military hardware more "transparent, smooth, efficient and less vulnerable to unethical practices".

"Adequate defence preparedness is critically dependent on sound defence acquisition policies," Singh said in Gurgaon, a satellite city of New Delhi, where he laid the foundation stone for India's first defence university.

"We have paid close attention to this and have continually reformed those policies," he said. "We will continue to seek the highest standards of probity in defence acquisition."

The government said in March it planned to draft new arms procurement procedures in the wake of a corruption scandal involving a contract to buy Italian helicopters.

Public anger over alleged bribes paid by Italian company Finmeccanica to secure a $748 million contract for 12 helicopters forced New Delhi in February to order an investigation and stall the defence deal.

The probe into the huge chopper deal has already seen police raid the home of the former air force chief and a "preliminary enquiry" report has linked four firms, four Westerners and seven Indians to the bribery allegations.

Italian prosecutors suspect that kickbacks worth around 50 million euros ($64 million) were paid to Indian officials to ensure Finmeccanica's British unit AgustaWestland won the contract, according to Italian media reports.

New Delhi put payments to Finmeccanica on hold and threatened to cancel the deal if any wrongdoing was uncovered. India has already received three of the choppers. The rest were to be delivered by the end of 2014.

The Congress party, up for re-election in May next year, has been hit by a string of scandals. Two ministers resigned this month, after one was accused of interfering in a graft probe and another linked to a bribery allegation.

The defence scandal erupted at a time when the government was already fighting off the national auditor's charges that its cut-price sale of telecom spectrum and allocation of coalfields cost the exchequer billions of dollars.

The controversy paralysed parliament and derailed measures to further open up the heavily state-controlled economy, as growth plunged to a decade-low of five per cent in the last financial year.

An opinion poll by the CNN-IBN television network released on Wednesday showed 67 per cent of respondents saying the government had lost its credibility due to multiple corruption scandals and 61 per cent saying Singh should exit.