WASHINGTON (AFP) - The United States said Friday it would raise concerns about trade, human rights and Syria during a closely watched visit by China's likely next leader next week, despite hopes to improve ties.

White House officials said they would seek to send a message to Vice President Xi Jinping that the United States welcomes China's rise, but that Beijing was testing the patience even of supporters of the relationship.

"China needs to recognize that it needs to continue to take steps to live up to the rules of the road that all nations abide by, particularly economically, in order to maintain support for the relationship in the United States," deputy national security adviser Ben Rhodes told reporters on a conference call.

With elections approaching in November the United States has been pressing China over trade policies seen as unfair, including what lawmakers call a disregard for intellectual property rights and an artificially weak currency.

Human rights groups say that China has also stepped up curbs on dissent, with dozens of government critics detained since last year. Democracy activist Zhu Yufu was sentenced Friday to seven years in prison, his wife said.

Residents say that China has imposed virtual martial law in Tibetan areas, amid a wave of self-immolations to protest Beijing's rule, and has kept tight control of the Uighur minority concentrated in northwestern Xinjiang region.

"It is an area of grave concern for us to witness the increase of tensions in Tibet and Xinjiang," said Danny Russel, President Barack Obama's top adviser on Asia.

"The US has spoken out about it, and we use every opportunity to urge the Chinese officials and leaders to exercise real restraint and to safeguard the human rights and the fundamental freedoms of all of Chinese citizens, including in Tibet," he said.

Russel dismissed fears of human rights groups that the United States would tone down comments to ensure a smooth visit by Xi, who will be welcomed Tuesday at the White House, State Department and Pentagon.

"This is an important part of our agenda and there's no reason that the conversations with Vice President Xi would depart from our longstanding practices," Russel said.

Separately Friday, State Department spokeswoman Victoria Nuland called on China to free Zhu, saying that the United States was "deeply concerned" that he was sentenced for subversion over writing a poem.

Xi's visit also comes after China joined Russia in vetoing a UN Security Council resolution that would have pressured President Bashar al-Assad of Syria to start a transition and halt what residents say is escalating violence.

"We'll continue to press that with the Chinese because, frankly, it's not, we believe, the right bet to believe that Assad is going to brutalize his people into submission," Rhodes told reporters on a conference call.

"We believe Assad's days are numbered and there needs to be a transition in Syria," he said.

Xi, who is expected to succeed President Hu Jintao next year, will also travel to Iowa and California as part of an effort to show a gentler side of China to an audience outside of Washington.

The 58-year-old vice president visited Iowa in 1985 on his first trip to the United States when he was a low-ranking official. He is said to have enjoyed his experience and will meet former associates in the Mississippi River town of Muscatine.

China's Vice Foreign Minister Cui Tiankai said Monday that Xi's visit aimed to bridge the "trust deficit" between the Pacific powers.