World leaders began arriving in Washington on Sunday for a two-day summit aimed at securing world's loose nuclear material.

The U.S. President Barack Obama invited a total of 46 heads of state and government and representatives to discuss ways to secure loose nuclear material around the world.

Before the summit which officially starts on Monday, Obama voiced his optimism that countries attending the summit would "make enormous progress" on both the commitment and specific steps to secure nuclear weapons and nuclear material.

"I feel very good at this stage in the degree of commitment and sense of urgency that I've seen from the world leaders" on confronting the possibility of a terrorist organization obtaining a nuclear weapon, Obama said.

He called the prospect of nuclear terrorism "the single biggest threat to U.S. security, both short term, medium term and long term."

"We think we can make enormous progress on this," Obama told reporters before he met with the South African President Jacob Zuma ahead of the summit.

The U.S. president later met with the Indian Prime Minister Manmohan Singh. The two leaders vowed to continue to strengthen the bilateral relationship and "looked forward to the upcoming U.S.-India Strategic Dialogue as the next step in that process."

They discussed development issues, the situation in Afghanistan and "a number of regional and global issues, including counterterrorism and nonproliferation."

The United States signed with India a controversial civilian nuclear cooperation pact in 2008, under which India agreed to separate its civilian and military nuclear facilities. India refuses to sign the Nuclear Non-Proliferation Treaty.

After Singh, Obama met with the Kazakh President Nursultan Nazarbayev. The two released a joint statement in which they pledged to intensify bilateral cooperation to promote nuclear safety and non-proliferation, regional stability in Central Asia.

During a meeting with the Pakistani Prime Minister Yousuf Raza Gilani, Obama noted that the two countries' multi-faceted and long-term strategic relationship "goes far beyond security issues."

Before the summit, advisers from participating countries have been working on two documents: a joint communique which will iterate commitments of the leaders, and a work plan on specific steps to "lock down the loose material in a very specific time frame."

Obama said Sunday that if organizations such as al-Qaida acquired nuclear weapons they "would have no compunction at using them."