US Secretary of State Hillary Clinton pushed for nuclear deals and deeper security cooperation with India on Tuesday as she visited the key US ally in the shadow of triple bomb blasts in Mumbai.

Clinton is on a three-day trip to India, lobbying for US commercial interests while seeking to balance the delicate relationships Washington maintains with violence-wracked South Asian countries.

She said she was "encouraged" by India and Pakistan's decision to restart their stop-start peace process, but she also heard Indian worries that a planned US troop drawdown in Afghanistan could lead to instability.

The top diplomat stressed that the US-India relationship, which President Barack Obama described as the "defining partnership of the 21st century", had made great progress in recent years, but was yet to fulfil its potential.

She singled out civil nuclear energy as an area where the countries "can and must do more" amid frustrations that private US nuclear energy firms are losing out in India to their state-owned French and Russian competitors.

Former president George W. Bush concluded a landmark energy pact with India in 2008 that lifted an embargo on selling atomic technology to New Delhi imposed after the country's first nuclear test in 1974.

Despite the diplomatic efforts of Washington to push through the deal, privately run US firms such as Westinghouse and General Electric have been unable to land contracts to build new reactors.

"Many of us worked very hard for that agreement, but we do expect it to be enforceable and actionable in all regards," Clinton said, voicing frustration that US firms still faced regulatory difficulties.

After triple blasts in Mumbai last week that left 19 dead, security was foremost among the subjects discussed in talks between Clinton, India's Foreign Minister S.M. Krishna and Prime Minister Manmohan Singh.

"We are deepening and expanding our efforts and making great strides together on behalf of counter-terrorism but also in respect of maritime security," she said at a news conference.

Key to security and stability in nuclear-armed South Asia is the relationship between India and Pakistan, which have fought three wars and remain deeply suspicious of each other.

Clinton said she was "encouraged" by the renewal of dialogue between New Delhi and Islamabad which was "so necessary for us to deal with the underlying problem of terrorism."

India and the US have repeatedly called on Pakistan to crack down on "safe havens" for militant groups which are thought to stage attacks on Indian soil and in Afghanistan.

"We do not believe that there are any terrorists that should be given a safe haven or free-pass by any government," she said.

Washington's own relations with Pakistan -- a crucial counter-terrorism ally -- have deteriorated since US commandos shot and killed Al-Qaeda leader Osama bin Laden on May 2 in a Pakistani garrison town, sowing distrust on both sides.

Krishna meanwhile reminded Clinton that India, which has spent billions of dollars on aid to Afghanistan, had much to lose by instability in the country as a result of the planned US troop withdrawal.

The United States has announced plans to withdraw 33,000 surge troops by the end of September 2012, with the first 10,000 due to depart this year.

"It is necessary for the United States to factor in Afghanistan's ground realities so that... Afghanistan will be in a position to defend itself against terrorism sponsored by the Taliban," Krishna said.

Clinton's two-day trip follows Obama's visit in November -- a courtship of India that reflects the rapid growth in the country's economy and a shift in power to emerging nations as a result of the global financial crisis.

Commerce has been booming, with bilateral trade up by 30 percent to nearly $50 billion in 2010.

Both countries also announced that they would resume technical-level negotiations on a bilateral investment treaty in August in Washington.