By Matt Spetalnick

WASHINGTON (Reuters) - China's "provocative" actions in maritime disputes with its neighbors are straining ties with the United States, raising questions over how the world's two biggest economies can work together, a senior U.S. official said.

The strong comments from Washington on Thursday come after deadly anti-China riots broke out in Vietnam in response to China towing an oil rig into a part of the South China Sea claimed by both Hanoi and Beijing.

"This is raising some fundamental questions for us about China's long-term strategic intentions," the U.S. official told Reuters, speaking on condition of anonymity. He said Beijing's move appeared to fit a "pattern" of advancing territorial claims through coercion and intimidation.

"China's activities are straining the U.S.-China relationship because it raises questions about our ability to partner together in Asia or even bilaterally."

The Vietnamese government says one person was killed in the anti-China violence on Tuesday and Wednesday, but a doctor at a hospital near one area of rioting said he had seen 21 dead bodies and that at least 100 people were wounded. An eyewitness to fighting between Chinese and Vietnamese workers in an industrial zone in the same area said she had seen at least 13 bodies and had heard of two other people killed.

There were no immediate reports on Friday of further violence.

Washington is in close contact with the Vietnamese government on "how most effectively to manage" Hanoi's standoff with Beijing, the U.S. official told Reuters,

The Philippines, one of Washington's closest allies in Asia, has said China is reclaiming land on a reef in the oil- and gas-rich South China Sea that both countries claim and is building what appears to be an airstrip on it. It has offered the United States the use of an underdeveloped naval base on a nearby island to ensure U.S. warships can enter the vicinity.

Vice President Joe Biden and other top U.S. officials told visiting General Fang Fenghui, chief of general staff in China's army, that Beijing's behavior in the maritime disputes was "dangerous and provocative" and must stop, the official said.

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The renewed tensions in the South China Sea underscore one of the biggest challenges in Asia facing President Barack Obama, who is under pressure by America's allies to accelerate a "pivot" of military assets to the region to counter China's rising influence.

In Washington on Thursday, Fang defended the deployment of the oil rig in the disputed South China Sea and blamed Hanoi, saying China cannot afford to "lose an inch" of territory.

The general also pointed the finger at Obama's strategic shift toward Asia, saying it had encouraged countries such as Vietnam, Japan and the Philippines to make trouble with Beijing. Vietnam and China are now grappling with one of the worst breakdowns in relations since the neighbors fought a brief border war in 1979.

WRONG LESSONS

The U.S. official dismissed Fang's accusation as "a fundamental mis-reading" of U.S. strategy and said China's assertiveness would only encourage Asian countries to seek greater U.S. involvement in regional diplomatic, economic and military affairs. Washington insists its Asia-Pacific re-engagement is not meant to contain China's rise but that Beijing must conduct itself according to international norms.

"We're concerned that China has learned the wrong lessons from Russia and Ukraine and has decided that unilateral assertion is the way to advance China's interests," the official said.

Russia's annexation of Ukraine's Crimea peninsula - and perceptions of limited U.S. options to get Moscow to back down - have heightened unease in parts of Asia over whether Beijing will be emboldened to use force to pursue its territorial claims in the East and South China Seas.

In Beijing, the official Xinhua news agency said China's foreign minister condemned Vietnam in an urgent phone call with the Southeast Asian country's deputy prime minister over the anti-Chinese protests.

Foreign Minister Wang Yi told Vietnamese Deputy Prime Minister and Minister of Foreign Affairs Pham Binh Minh in the call on Thursday night that "Vietnam bears unshirkable responsibility for the violent attacks against Chinese companies and nationals", the report said.

Vietnamese Prime Minister Nguyen Tan Dung has called on police and state and local authorities to restore order and ensure the safety of people and property in the affected areas.

Foreign Ministry spokesman Le Hai Binh confirmed one death in the clashes, and described media reports and accounts on social networking sites of higher casualties as "groundless".

China's state news agency Xinhua reported that at least two Chinese nationals had died and more than 100 were hospitalized.

The worst violence appears to have taken place in an industrial zone in the central province of Ha Tinh. Journalists' movements are restricted in tightly controlled Vietnam, where the government maintains an iron grip on law and order, and it was not possible to get independent confirmation of the death toll.

The doctor at the Ha Tinh hospital who had seen 21 dead bodies and the witness who saw the fighting in the industrial zone did not want to be identified by name.

(Corrects eyewitness account of deaths during riot in paragraph 5)

(Additional reporting by Nguyen Phuong Linh and Martin Petty; Writing by Raju Gopalakrishnan; Editing by Simon Cameron-Moore and Mark Bendeich)