"It has to prove that the government is doing all that is possible (to bring the perpetrators to book)," he said.

"It is the least they should do to convince us of their sincerity," he added.

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The prime minister was responding to Pakistani President Asif Ali Zardari's suggestion that the two countries resume the composite dialogue process that has been frozen in the wake of the Mumbai attacks that India has blamed on elements operating from across the border.

Expressing his sympathies to Pakistan over Monday's terror attack on a police academy near Lahore, Manmohan Singh said both countries should jointly take on terrorists.

"I have always said that India and Pakistan have to face jointly the scourge of terrorism," he said.

"I sincerely wish the (Pakistani) government will have the courage to take up the challenge."

"Our sympathies are with the government and people of Pakistan," he said in his first remarks on Monday's terror attack on a police training school near Lahore that left 18 people dead.

The prime minister said he looked forward to meeting US President Barrack Obama on the sidelines of the G20 summit in London on Thursday.

"We will be talking about bilateral and global issues, climate change and terrorism," he said.

As for the summit itself, the prime minister said: "The agenda should be to put the global economy on the path of sustainable growth. Then, economies like India that were hoping for eight to nine percent growth (before the meltdown) will be able to return to that path.

Manmohan Singh also termed as "very unfortunate" the hate remarks attributed to Bharatiya Janata Party (BJP) candidate Varun Gandhi.

"If what has been attributed to him is true, then it is very, very unfortunate," the prime minister said.

"He has a distinguished legacy. Right from the time of (India's first prime minister) Jawaharlal Nehru, his (Nehru-Gandhi) family has devoted their life to the cause of secularism.

"I speak more in sorrow than in anger," the prime minister said, the second time he has commented on the subject.

Varun Gandhi, the 29-year-old grandson of former prime minister Indira Gandhi, courted controversy over his reported speeches, which he claims were doctored, in March in Pilibhit in Uttar Pradesh where the BJP has named him its candidate for the Lok Sabha elections. The purported speeches are virulently anti-Muslim.

The Election Commission reprimanded Varun Gandhi while the police lodged a criminal case against him for the incendiary remarks. A case under the National Security Act was later registered and he remains behind bars.