The skies between China and the United States are getting busier.

In 2015, 75 weekly nonstop flights were added between the two countries - up more than 30 percent from 2014 - and more are on the way.

Six of the eight airlines that provide direct US-China services opened new routes in 2015, adding to their fast-growing schedules. Some even expanded service to China's second-tier cities.

During the past peak season, there were 320 weekly flights, according to information provided by the airlines.

The four largest Chinese airlines - Air China, China Eastern, China Southern and Hainan Airlines - expanded in the US market significantly in 2015.

Leading China's "Big Four" with 55 weekly flights from Beijing to seven US cities, Air China started a new Beijing to Newark, New Jersey, route in October and increased the frequency of the Beijing-Los Angeles route to 21 flights a week.

Hainan Airlines opened three new routes - Shanghai to Boston, Shanghai to Seattle, and Beijing to San Jose, California - becoming the first airline to fly nonstop from China to the heart of Silicon Valley.

China Southern started a direct route in June between San Francisco and its hub, Guangzhou, after seeing strong demand for such flights via Wuhan, a second-tier city, since that route started in December 2014.

China Eastern is the first airline to serve Nanjing, another second-tier Chinese city, with a new route from Chengdu to Los Angeles via Nanjing.

Delta Air Lines, moving beyond its hubs in Seattle and Detroit since July, has operated daily flights from Los Angeles to Shanghai, one of the most popular routes. American Airlines added Dallas-to-Beijing flights and began to upgrade planes flying between the two countries to new Boeing 787s.

Thirteen US airports currently have nonstop flights to seven Chinese cities. San Francisco is connected to six cities in China, Los Angeles to five, New York to four, and the rest to one or more among Beijing, Hong Kong and Shanghai.

"The West Coast now has new routes to cities like Guangzhou, Chengdu, Wuhan and Nanjing," said Luo Linquan, the Chinese consul general in San Francisco. "The great weather in California makes it a well-liked travel destination all year long. Tourists fly into San Francisco to see the great schools and Silicon Valley. They go to Los Angeles for Hollywood and Disneyland."

"One of the fruits of President Xi Jinping's state visit is the establishment of US-China Tourism Year. We will expand state-province collaborations to promote tourism," Luo said. "Tourism between China and the US will grow as the relationship between the two countries deepens. There's a lot of potential."

He hopes to see more Chinese translators at airports, Chinese menus in restaurants and Chinese signs at tourist sites.

United Airlines announced a plan to start a San Francisco to Xi'an route this year.

China Eastern will add a Shanghai-Chicago route, and Hainan Airlines will fly nonstop from Changsha to Los Angeles, making it the first airline to connect Changsha with North America.

China will add at least 200 international air routes by the end of next year, according to Li Jiaxiang, head of the Civil Aviation Administration of China. It currently has 663 international routes operated by domestic and foreign carriers, of which 369 were launched last year.