Thousands of overseas Chinese, waving Chinese and US national flags, gather in downtown Washington to welcome the arrival of visiting President Xi Jinping on Thursday. ZHANG WEIRAN/CHINA NEWS SERVICE

Wearing a black suit, white shirt and no tie, President Xi Jinping took a casual walk side by side with US counterpart Barack Obama along the West Wing of the White House on Thursday night before crossing Pennsylvania Avenue for a three-hour private dinner at Blair House, the state guesthouse.

This was the third time the two chatted like old friends without the top-level diplomatic rituals that put distance between them, though hard topics like the economy and cybersecurity lay ahead.

Before their walk, Obama offered Xi a warm welcome at the White House, greeting his Chinese counterpart with "Nihao".

Officials hope that by starting with a casual meeting, the leaders of the world's preeminent military and economic powers will find time for a less-staid exchange of views.

Fan Jishe, a US studies researcher at the Chinese Academy of Social Sciences, said that a private, informal talk between Xi and Obama will help nurture personal trust and give them a chance to exchange visions of each country's strategic development, which has been a key concern for many people.

"We don't usually say this, but in the past the top leaders' meetings were much more formal and sometimes ceremonial, during which personal relationships were not easily established," Fan said.

"The current small-scale and face-to-face meetings allow the two leaders to talk about matters frankly and sincerely, especially regarding some topics that are not likely to be raised during formal political meetings," Fan added.

Senior Obama national security aide Ben Rhodes agreed. "Far and away, the most constructive engagements they've had have been in their private dinners," he said.

Thursday's informal dinner is an opportunity, Rhodes said, to "put aside the talking points and actually get a window into one another's world view".

Aside from the closed-door dinner with Obama, at least two other prominent dinners were waiting for Xi in Washington.

US Vice-President Joe Biden and Secretary of State John Kerry will offer Xi and his wife, Peng Liyuan, a welcome luncheon in the State Department on Friday.

Xi's evening that day is reserved for an official White House state dinner with Obama and his wife, Michelle Obama, featuring Maine lobster. The lobster will be poached in butter and served with traditional rice noodle rolls embedded with spinach, shiitake mushrooms and leeks when Obama and his wife welcome Xi and Peng to dinner in the East Room with a few hundred guests.

When then-president Hu Jintao was the guest in 2011, the lobster was also poached and served with glazed carrots and black trumpet mushrooms.

Agence France-Presse and Associated Press contributed to the story.