US President Barack Obama opened a meeting with his Chinese counterpart Xi Jinping with a promise of "candid" discussion over Beijing's suspected military buildup in the South China Sea.

"We will have candid exchanges about areas where we have differences, issues like human rights, cyber and maritime issues," Obama said as he sat across the conference table from his Chinese counterpart.

US officials have expressed concern that China's actions in the South China Sea are not consistent with Xi's pledge at the White House last year not to pursue militarization of the hotly contested and strategically vital waterway.

China claims virtually all the South China Sea despite conflicting claims by Brunei, Malaysia, Vietnam, Taiwan and the Philippines, and has built up artificial islands in the area in recent months, including some with airstrips.

"We do continue to be concerned about militarization in the South China Sea," said senior Obama foreign policy aide Ben Rhodes ahead of the meeting with Xi.

"We certainly have seen developments, reports that are not consistent with commitments to avoid and to non-militarize the South China Sea."

"This will be certainly an important topic of conversation between the two presidents."

Washington has since October carried out two high-profile "freedom of navigation" operations in which it sailed warships within 12 nautical miles of islets claimed by China.

Xi spoke of the need to avoid misunderstandings and big disruptions in the "major power relations" between the two countries.

- Keeping up pressure on Pyongyang -

Obama and Xi were also set to discuss pressure on North Korea, which in January detonated a nuclear device and launched a long-range rocket a month later, prompting UN sanctions backed by both Beijing and Washington.

"Of great importance to both of us is North Korea's pursuit of nuclear weapons, which threatens the security and stability of the region," said Obama.

"President Xi and I are both committed to the denuclearization of the Korean peninsula and the full implementation of UN sanctions."

"We are going to discuss how we can discourage actions like nuclear missile tests that escalate tensions and violate international obligations."

The White House wants to keep up pressure on the North Korean regime, increasing the economic and diplomatic cost of ignoring international appeals to mothball its nukes.

US officials have long believed that China could more forcefully wield its influence over Pyongyang, including encouraging its Stalinist neighbor to tone down destabilizing rhetoric.

Tensions are only expected to rise in advance of a major Communist Party Congress in North Korea in May.

The White House has begun talks with South Korea over the deployment of a missile defense system.

But it has struggled to convince China that the move is only in response to Pyongyang's threats.

China fears it may be an effort to deepen US influence in the region.

"It is designed and capable only of responding to the North Korean threat," said Dan Kritenbrink, senior Asian affairs director at the National Security Council.

"It in no way threatens either Chinese or Russian or other security interests in the region and will do nothing to undermine strategic stability between the United States and China."