With a combative opposition accusing him of going soft on 26/11 terror, Prime Minister Manmohan Singh Saturday said he was optimistic about the resumed peace process with Pakistan, but stressed that he has made it clear that if another "barbarous" Mumbai attack were to happen, it will be a "setback".

"I did discuss (with Pakistani Prime Minister Yousuf Raza Gilani) the Mumbai terror attack. Those who perpetrated the barbarous attack must be brought to justice," Manmohan Singh told reporters on board his special aircraft while returning from the SAARC summit in the Maldives.

"I left Prime Minister Gilani in no doubt that if public opinion in India is not satisfied that justice is being done to those responsible to the barbarous attack, it won't be possible to move forward with the peace process," said Manmohan Singh.

"We both recognize that if there is another attack like Mumbai, it will be a setback to the normalization of relations. And that was understood by Prime Minister Gilani," said the prime minister.

The prime minister's clarification came two days after he held wide-ranging talks with his Pakistani counterpart on the sidelines of the SAARC summit and amid the Bharatiya Janata Party's accusation that he has gone soft on terror emanating from Pakistan barely days before the third anniversary of 26/11 attack.