China has issued a travel warning for the United States, saying Chinese visitors have been interrogated, interviewed and subjected to other forms of what it called harassment by U.S. law enforcement agencies.

The warning issued Tuesday urges Chinese citizens and Chinese-funded bodies in the U.S. to step up their safety awareness and preventative measures and respond "appropriately and actively." It was issued by the Foreign Ministry, and the Chinese Embassy and consulates in the U.S.

The warning comes amid an increasingly bitter trade dispute between the sides and tougher immigration enforcement by the Trump administration.

Also Tuesday, the Ministry of Culture and Tourism issued its own travel alert for the U.S., noting the high frequency of shootings, robberies and theft in the country. Chinese students abroad were also urged Monday to assess the risks involved given tightened visa restrictions.

The alerts are valid until Dec. 31.

Commenting on the warning issued to students, Foreign Ministry spokesperson Geng Shuang said the U.S. side had to be aware of the obstacles and impact they have created.

"For some time, the U.S. has politicized normal China-U.S. educational exchanges using the China threat and Chinese infiltration as excuses," Geng said at a daily briefing.

Chinese travel to U.S. falling

Chinese social media has been abuzz with talk of U.S. visa and green card denials and other travel, education and employment woes under Trump, who has sought to restrict the numbers of foreign visitors and others coming to the U.S. for what he claims are security and economic reasons.

One who did receive a visa, although not without a lengthy wait, was Cao Yue, a senior student majoring in aeronautics at Beijing-based Beihang University.

After being admitted to the master's program in mechanical engineering and robotics at Columbia University, she applied for a visa at the U.S. Embassy in Beijing and was told it would take weeks to process.

Cao said she was asked questions about her parents' profession and what specific types of robots she planned to work on.

"I was expecting [to wait]. I knew what was going to happen. This major involves a sensitive field," said Cao, who received her visa after about six weeks.

Chinese travel to the U.S. is falling after more than a decade of rapid growth, leaving cities, malls and other tourist spots scrambling to reverse the trend.

Travel from China to the U.S. fell 5.7 per cent in 2018 to 2.9 million visitors, according to the National Travel and Tourism Office, which collects data from U.S. Customs forms. It was the first time since 2003 that Chinese travel to the U.S. slipped from the prior year.

China also issued a travel warning for the U.S. last summer, telling its citizens to beware of shootings, robberies and high costs for medical care. The U.S. shot back with its own warning about travel to China.