U.S. President Barack Obama on Sunday greeted world leaders who began to arrive for a summit aimed at securing loose nuclear material around the world, meeting four of them and calling another ahead of the two day meeting that 's scheduled to begin on Monday, officials said.

Among the leaders Obama was set to host in a series of bilateral meetings on Sunday, Indian Prime Minister Monmohan Singh and Pakistani Prime Minister Syed Yusuf Raza Gilani stood out. The two South Asian countries were both nuclear-armed, and were arch rivals in the region.

Obama started his bilateral meetings in the afternoon, first meeting with Singh. Secretary of State Hillary Clinton, Secretary of Energy Steven Chu, White House Chief of Staff Rahm Emanuel and National Security Advisor James Jones were present at the meeting, according to the White House.

On the Indian side, Foreign Secretary Nirupama Rao and National Security Advisor Shivshankar Menon attended the meeting.

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

After meeting with Singh, which took place at the Blair House, across the street from the White House, Obama will meet with Kazakhstan's President Nursultan Nazarbayev and South African President Jacob Zuma.

Obama's last scheduled bilateral meeting of the day was with Gilani. The two countries concluded their first ever high-level strategic dialogue last month. Islamabad reportedly presented a "wish list" to the White House ahead of the dialogue, asking Washington to play a constructive role in the Pakistan-India peace process and to offer a series of assistance ranging from military equipment to nuclear cooperation.

The Obama administration reaffirmed last December its commitment to Islamabad by helping Pakistan address immediate energy, water and economic crisis.

Obama is scheduled to call Nigeria's Acting President Goodluck Jonathan later in the day.

The Nuclear Security Summit is to begin on Monday evening, the United States is hoping leaders and representatives from 47 countries can agree on specific steps to make nuclear material more secure, preventing terrorists from acquiring nuclear materials and weapons.