(AP Photo/Damian Dovarganes)

LOS ANGELES, Calif. - Chinese Vice-President Xi Jinping's pivotal four-day visit to the United State brings him and U.S. Vice-President Joe Biden on Friday to Los Angeles, where the vast trade imbalance between the two countries is at its starkest.

The tour by China's soon-to-be leader comes at a politically challenging time in U.S.-China relations, with the White House sending stern messages on currency and trade policies and Republican presidential candidates claiming that President Barack Obama isn't doing enough to keep America competitive with the Chinese economy.

The Asian power sells four times as many goods to the U.S. as the United States sends in return to China, and much of that trade pours through the giant Port of Los Angeles, the busiest port in the U.S.

Xi met with California Gov. Jerry Brown on Thursday and toured a shipping terminal at the port.

Nearly 60 per cent of the imports moving through the Port of Los Angeles come from China, including $120 billion worth of computers, TVs, sneakers and other goods last year. The U.S. shipped $13.5 billion in exports to China through the port last year.

On Friday, Biden and Xi start with a China trade forum in Los Angeles, followed by a luncheon, a school visit to meet children learning Mandarin, then a governor's forum at Disney Hall.

In a carefully scripted event, Xi took a short walking tour through the China Shipping terminal with Brown and Mayor Antonio Villaraigosa. The facility sprawls over nearly 100 acres (40 hectares).

"We're not just growing our ports, but we're greening our ports," Villaraigosa told Xi.

"When I heard that this is an environmentally friendly green port, I felt that this was a major achievement," Xi later told a crowd in a brief statement. "This is a solid foundation for future U.S.-China trade and economic co-operation."

As with his previous travels, Xi was focusing on forging relationships.

Xi spent the morning Thursday in Iowa, where officials from the U.S. and China signed a five-year deal to guide discussions on food security, food safety and sustainable agriculture.

China became the top market for U.S. agricultural goods last year, purchasing $20 billion in U.S. agricultural exports, according to the U.S. Department of Agriculture.

Much of Xi's visit, which began earlier this week in Washington, D.C., has been focused on agriculture. The strategic co-operation agreement signed Thursday outlines mutual goals and responsibilities of each nation.

"It charts the course and gives us a guiding document that we can reference and, over time, refine and improve," said Scott Sindelar, the agricultural minister counsellor at the U.S. embassy in Beijing, who attended the Des Moines conference.

According to the USDA, the value of U.S. farm exports to China supported more than 160,000 American jobs last year.

U.S. Secretary of Agriculture Tom Vilsack said the two nations will have to work together to help feed a growing global population.

"We have the responsibility and opportunity to work together to address the causes of global hunger that affect more than 925 million people. Current populations trends mean that we must increase agricultural production by 70 per cent in the year 2050 to feed nearly 9 billion people," he said.

Not everyone celebrated the vice-president's arrival. The California Fair Trade Coalition, a San Francisco-based non-profit that supports expanding trade while promoting economic justice, issued a statement calling on Brown to "address China's predatory trade practices."

"The economic potential for trade with China is massive, but if they aren't forced to level the playing field, this can only be a losing proposition for U.S. workers," said coalition director Tim Robertson.