The manner in which India and Pakistan have pursued atomic weapons has "upset the balance of nuclear deterrence", Secretary of State Hillary Clinton on Saturday said, asserting the US is working hard with both countries to try to limit their number of nuclear stockpiles.

"There are three pillars to the Non-Proliferation Treaty. One is disarmament, one is nonproliferation, and one is the peaceful use of nuclear weapons, the peaceful use of nuclear energy for civil nuclear purposes," Clinton said in her speech on nuclear non-proliferation at the University of Louisville.

"So the United States will continue to demonstrate its willingness, in concert with Russia, because we have so many more weapons than any of the other countries -- you know, by a very, very big margin," she said in her speech on 'No Greater Danger: Protecting our nation and allies from nuclear terrorism and nuclear proliferation', in Kentucky.

"Other countries that have pursued nuclear weapons - like India and Pakistan, for example -- have done so in a way that has upset the balance of nuclear deterrence," Clinton said.

"And that's why we're working with both countries very hard to try to make sure that their nuclear stockpiles are, you know, well tended to, and that they participate with us in trying to limit the number of nuclear weapons. And both of them will be in Washington next week," she said.

Prime Minister Manmohan Singh would represent India at the Nuclear Security Summit next week convened by US President Barack Obama, which is being attended by more than 40 world leaders including Pakistani Premier Syed Yusuf Raza Gilani.