Seoul: US President Barack Obama on Tuesday voiced concern over safety of Pakistan's nuclear arsenal, saying the world cannot allow non-state actors and terrorists to get their hands on the nuclear weapons and end up destroying cities.

"We can't afford to have non-state actors and terrorists to get their hands on nuclear weapons that would end up destroying our cities or harming our citizens," Obama told reporters alongside Pakistani Prime Minister Yousuf Raza Gilani before the two leaders held private talks on the sidelines of the nuclear security summit here.

The West is concerned over the safety of Pakistan's nuclear weapons as it remains vulnerable because the atomic facilities are located in areas where "Taliban and al-Qaeda are more than capable of launching terrorist attacks".

In their first meeting since the killing of Osama bin Laden in a covert US raid on Pakistani soil last May, the two leaders tried to rescue a troubled anti-terror alliance which has been full of mistrust and recriminations in recent times.

The bilateral ties plunged to an all time low in November when a cross-border NATO air raid killed 24 Pakistani soldiers, prompting Islamabad to curtail ties with the US and launch a parliamentary debate on new terms of engagement with Washington.

"There have been times - I think we should be frank – in the last several months where those relations have experienced strains," Obama told reporters.

Obama said it was important for the both countries to have candid and open talks.

Obama said he expects Pakistan's review of bilateral ties will result in a "balanced approach that respects Pakistan's sovereignty but also respects our concerns with respect to our national security and our needs to battle terrorists who have targeted us in the past."

During the debate on new terms of engagement with the US, angry Pakistani lawmakers have demanded an American apology and taxes on NATO convoys into Afghanistan.

"We want to work together with you," Gilani told Obama, in an effort to rebuild the strained Pak-US ties.

The two leaders also expressed a desire to stabilise and secure the situation in the war-torn Afghanistan.

"I also wanted to express to the Prime Minister my appreciation for his recognition that it's in both of our interests, and indeed in all of our interests, to see an Afghan-led reconciliation process that needs to take place," Obama said.

On his part, Gilani said: "Pakistan wants stability in Afghanistan. If there is stability in Afghanistan, there is stability in Pakistan. And we both, Afghanistan and Pakistan, want to work together with you for peace, prosperity and progress across the world."

"We are committed to fight against extremism and terrorism. It is in the interest of Pakistan, for a stable, peaceful, prosperous, independent and sovereign Pakistan," he said.