US Secretary of State Colin Powell has called for a nuclear proliferation network led from Pakistan to be completely rooted out.
Mr Powell was speaking in Delhi, at the start of a tour of South Asia.
It is his first visit to the region since Pakistan's top nuclear scientist AQ Khan admitted leaking nuclear secrets to Iran, Libya and North Korea.
Mr Powell will travel to Pakistan on Wednesday where the nuclear issue and Kashmir will be high on the agenda.
Pakistan and India have fought two wars over Kashmir
Speaking after meeting Indian Foreign Minister Yashwant Sinha, Mr Powell said Pakistani President Pervez Musharraf was "as determined as we are" to put an end to nuclear proliferation.
"But we can't be satisfied until the entire network is gone, branch and root," he said.
The secretary of state also said infiltration of militants into Indian-administered Kashmir from Pakistan-administered Kashmir was dropping.
"We will be watching that it stays that way."
In landmark talks in January between Indian Prime Minister Atal Behari Vajpayee and General Musharraf the two countries agreed to discuss the contentious Kashmir issue, which both countries claim.
Correspondents say Mr Powell will encourage both leaders to continue the process during his meetings with them.
Over the weekend Delhi and Islamabad exchanged a fresh war of words over the disputed state.
India rejected a statement by President Musharraf that Kashmir was the "central issue" dividing the two countries.
Mr Powell is also expected to hold talks on strengthening Indian-US ties, including joint space programmes and high-technology trade.
In Pakistan, Mr Powell is expected to discuss the hunt for al-Qaeda leader Osama Bin Laden.
He will also pay a short visit to Kabul to reaffirm US support for Afghan President Hamid Karzai, as US troops begin a fresh offensive in the country to capture al-Qaeda and Taleban leaders.
Mr Powell led international mediation efforts to prevent war between India and Pakistan as both sides built up huge forces along their common border after armed gunmen attacked the Indian parliament in Delhi in December, 2001.