At a time of heightened tensions between nuclear-armed rivals India and Pakistan, US President Barack Obama urged Pakistan on Thursday to avoid developments in its nuclear weapons programme that could increase risks and instability.

In talks with Pakistani Prime Minister Nawaz Sharif at the White House, Obama also sought help in getting the Afghan Taliban back to peace talks, something vital to his faltering bid to bring US troops back from Afghanistan.

With tensions high between Pakistan and India, Washington has been concerned about Pakistan's development of new nuclear weapons systems, including small tactical nuclear weapons, and has been trying to persuade Pakistan to make a unilateral declaration of "restraint."

However, Pakistani officials said Islamabad will not accept limits to its weapons programme and argue that smaller tactical nuclear weapons are needed to deter a sudden attack by India.

In reference to Pakistan's nuclear weapons programme, Obama "stressed the importance of avoiding any developments that might invite increased risk to nuclear safety, security, or strategic stability," a White House statement said.

In a joint statement, both leaders said "all sides" should act with restraint and work toward strategic stability in South Asia.

The statement said Obama and Sharif expressed their commitment to the Afghan peace process and called on Taliban leaders to enter direct talks with Kabul, which have stalled since inaugural discussions in Pakistan in July.

The talks broke down after the Afghan intelligence agency said Taliban leader Mullah Omar had been dead for two years.

"It's a setback, no doubt, and it will take some time to overcome this setback, but we will try again," Sharif told reporters after his meeting with Obama.

In his talks with Sharif, Obama also raised concerns about Americans held hostage by militants in the region and welcomed Sharif's offer to assist in ensuring their safe return, the White House said in an apparent reference to an American couple kidnapped in Afghanistan in 2012.

A senior administration official declined to elaborate. "For their safety and security, we are not going to offer specific details beyond the fact that we are aware of a small number of American hostages in this region," the official said, speaking on condition of anonymity.

The Taliban insurgency has escalated since tens of thousands of US-led NATO combat troops withdrew from Afghanistan ahead of an end-2014 deadline, hampering Obama's efforts to withdraw remaining US troops.

Michael Kugelman of the Wilson Center think tank said that as well as seeking help to revive the peace talks, Obama would have stressed to Sharif the need for Pakistan to do away with militant sanctuaries inside its borders used as bases from which to target the US-backed Afghan government and US forces.

"Obama knows that a political solution is needed to end the Afghan war, and for that you need help from the Pakistanis," he said.However, Bruce Riedel of the Brookings Institution said it was not clear Sharif had the clout with his own army to get military leaders to pressure the Taliban back into talks.

Despite the tensions, the Obama administration is preparing to sell Islamabad eight F-16 fighter jets in a bid to bolster ties, a US source familiar with the matter said.

The joint statement made no mention of the sale, which Congress could block, but said Sharif "expressed satisfaction with the cooperation achieved in defence relations."

The joint statement said the two leaders discussed the continuing threat of nuclear terrorism and stressed the importance of improvement in Pakistan-India relations.

The Federation of American Scientists said this week that since 2011, Pakistan has deployed two new nuclear-capable short-range ballistic missiles and a new medium-range ballistic missile and was developing two extended-range nuclear-capable ballistic missiles and two nuclear-capable cruise missiles.

It estimated Pakistan's stockpile had grown to 110 to 130 warheads from 90 to 110 in 2011 and could reach 220 to 250 by 2025, making it the world's fifth-largest nuclear-weapons state.

Washington worries that the smaller size of tactical nuclear weapons makes them more tempting to use in a conventional war - and harder to prevent from falling into militant hands.

Pakistani officials say Washington is demanding unreasonable limits on its nuclear weapons while not offering much in return apart from a hazy promise to consider Pakistan as a recognised recipient of nuclear technology.