Foreign students in the United States - numbering in the hundreds of thousands - will have to leave the country if their classes are all taught online this autumn or if they transfer to another school with in-person instruction, a government agency has said.

It is not immediately clear how many student visa holders will be affected by the move, but foreign students are a key source of revenue for many US universities as they often pay full tuition.

China ranked first among countries of origin for international students in the US with nearly 370,000 during the 2018-2019 academic year, according to data published by the Institute of International Education.

The US Immigration and Customs Enforcement (ICE) agency said it would not allow holders of student visas to remain in the country if their school was fully online for the autumn.

Those students must transfer or leave the country, or they potentially face deportation proceedings, according to the announcement.

The ICE guidance applies to holders of F-1 and M-1 visas, which are for academic and vocational students.

The State Department issued 388,839 F visas and 9,518 M visas in the 2019 financial year, according to the agency's data.

Chinese Foreign Ministry spokesman Zhao Lijian told reporters in Beijing that China was closely following the policy changes in the US and would do everythingto protect the rights and interests of Chinese students.

US colleges and universities have begun to announce plans for the autumn 2020 semester amid the coronavirus pandemic.

Harvard in Massachusetts announced that it would conduct course instruction online for the 2020-2021 academic year.

The guidance does not affect students taking classes in person. It also does not affect F-1 students taking a partial online course-load, as long as their university certifies the student's instruction is not completely digital.

M-1 vocational programme students and F-1 English language training programme students will not be allowed to take any classes online.

The Trump administration has imposed a number of new restrictions on legal and illegal immigration in recent months as a result of the pandemic.

In June, the administration suspended work visas for a wide swath of non-immigrant workers that it argued compete with US citizens for jobs.

The administration has also effectively suspended the admission of asylum seekers at the southern border with Mexico, citing health risks as justification.

New York extends quarantine rules to three more US states

It comes as visitors from three more US states who travel to New York will be required to quarantine for 14 days to control the spread of coronavirus, Governor Andrew Cuomo said on Tuesday, as alarm grew over a surge in infections in large parts of the country. 

New York state, which had been the early epicenter of the US outbreak, unveiled the travel advisory last month in an effort to prevent a resurgence after the state got its outbreak under control. 

New York Governor Andrew Cuomo

Delaware, Kansas and Oklahoma, all of which are grappling with "significant" community spread of the virus, have been added to the list, Governor Cuomo announced in a statement, bringing the total number of states under the travel advisory to 19. 

New Jersey, which along with New York had experienced the brunt of the early part of the US outbreak, also added the three states to its quarantine order. 

Parts of the United States, including Florida, Texas and California, have experienced a sharp rise in infections in the past two weeks, an indication that the pandemic remains largely uncontrolled despite the end of lockdowns to control its spread. 

New Covid-19 infections are rising in 42 states, based on a Reuters analysis of cases for the past two weeks compared to the prior two weeks, putting the Unites States close to 3 million total cases. 

More than 130,000 Americans have died of Covid-19 - about a quarter of the global total - and the US Centers for Disease Control and Prevention has forecast that the death toll could reach 160,000 later this month. 

The surge has forced authorities to backpedal on moves to reopen businesses, such as restaurants and bars, after mandatory lockdowns in March and April reduced economic activity to a virtual standstill and put millions of Americans out of work. 

Miami, one of the current hot zones for the rise in new infections, has imposed a mask wearing requirement and ordered some businesses to close, with penalties imposed on those who do not comply. 

More states are reporting a troubling increase in the percentage of Covid-19 diagnostic tests coming back positive - a key indicator of community spread that experts refer to as positivity rate. 

Two dozen states, mostly in the South and West, have averaged positivity rates over the past week exceeding 5%, a level the World Health Organization considers to be concerning, data collected by Reuters shows. 

California's positivity rate has also risen over the past two weeks. But Governor Gavin Newsom on Monday cited a 50% two-week spike in hospitalisations as impetus for beefed-up enforcement actions during the recent Fourth of July holiday.