US President Barack Obama's drive to build relations with both India and Pakistan faced a delicate test Sunday as he began meetings with the leaders of the nuclear-armed rivals to discuss security issues.

The president, accompanied by US Secretary of State Hillary Clinton, strode across Pennsylvania Avenue to Blair House to hold talks with Indian Prime Minister Manmohan Singh on the eve of a 47-nation summit on improving nuclear security.

Obama and Clinton were also to meet with Pakistani Prime Minister Yousuf Raza Gilani later in the day.

But no meeting has been scheduled between Singh and Gilani. The two nations cautiously resumed talks in February which had been cut off after the deadly Mumbai assault in 2008.

Robert Blake, the assistant secretary of state for South Asia, said that the "pace, scope and character of relations" between India and Pakistan was up to the two countries.

But he added: "The United States hopes that India and Pakistan can improve relations between two friends of the United States."

Obama invited Singh in November for the first state dinner of his presidency, an honor meant to push forward a decade-old drive to transform the world's two largest democracies into partners.

The Obama administration has at the same time welcomed Pakistani actions against Islamic extremists and tried to curb widespread anti-Americanism in the country by seeking cooperation on issues beyond Afghanistan.

At a first-of-a-kind strategic dialogue with the United States last month, Pakistan presented one item on its wish-list: a US role in Kashmir, the divided Himalayan territory at the heart of two wars with India.

The United States has publicly ruled out mediation over Kashmir, which India considers a domestic issue. But some supporters of India have worried the Obama administration may put subtle pressure on New Delhi.

Edward Burrier, an adviser to Republican Congressman Ed Royce, said that the United States should devote its energy to fighting Lashkar-e-Taiba (LeT), the Islamist movement blamed for the Mumbai bloodbath.

"I'm sympathetic to the view that to reduce troop tensions on the India-Pakistan border, you can't leave Kashmir hanging out there forever," Burrier said.

"But it seems the best way to reduce tensions between the two is pressing on Pakistan to permanently arrest the LeT's leadership and to crack down on the organization, not lean on New Delhi," Burrier said.

Blake said he encouraged Pakistan to crack down on Lashkar-e-Taiba during a visit in March, while praising the government's actions against Islamic militants elsewhere.

Media in both India and Pakistan both routinely charge that the United States is slanted toward the other country.

Indian commentators have recently voiced outrage that US prosecutors reached a plea deal with David Headley, who surveyed targets in Mumbai before the siege that left 166 people dead.

Under the deal, Headley -- the US-born son of a former Pakistani diplomat and an American woman -- agreed to cooperate with US investigators in return for avoiding the death penalty and extradition to India.

Teresita Schaffer, director of the South Asia program at the Center for Strategic and International Studies think-tank, said the United States largely had different interests in India and Pakistan.

Since the late 1990s, "the US is much better able to insulate its policies towards India from those towards Pakistan," she said.

"But it's not 100 percent. In any two countries that have that pronounced a rivalry, there's going to be some concern in each country based on how you deal with the other," she said.

India and Pakistan, however, are both seen as crucial to the theme of Obama's summit -- preventing an attack from loose nuclear materials.

The South Asian nations gate-crashed the elite club of nuclear powers in 1998. While the United States has since praised India's nuclear security and pursued cooperation, it has feared proliferation from Pakistan.

"Pakistan has been a source of concern in the past," State Department spokesman Philip Crowley said. "But if we're going to strengthen the nonproliferation regime going forward, we want to see Pakistan invested in this process."

Premier Gilani said that Pakistan's nuclear weapons were safe as he left for the United States to attend the summit.

"I assure the world, I assure the people of Pakistan that the nuclear capability, the nuclear program is in safe hands," the Associated Press of Pakistan quoted Gilani as saying.