Choking smog blanketed Beijing and much of northern China Monday as climate change talks opened in Paris and a new Chinese report raised the alarm about rising sea levels.

As the global summit began, the US embassy in Beijing recorded concentrations of PM2.5, tiny airborne particles which embed deeply in the lungs, at 625 micrograms per cubic metre -- 25 times above the World Health Organisation's 25 microgram recommended maximum.

Plummeting visibility grounded flights and local authorities said levels in one southwestern district had reached 976 micrograms per cubic metre -- more than 39 times the WHO limit.

In the centre of the capital the air had an acrid taste, and skyscraper summits were invisible from the ground as a grey haze washed out colour.

"You can't even see people standing directly in front of you," wrote one disgruntled commuter on Chinese Twitter-equivalent Sina Weibo. "It feels like even the subway station is full of haze."

Multiple flights into the capital's second airport were cancelled, an airline said, with the city's main air hub adding more than 50 planes could not take off.

China is the world's largest emitter of greenhouse gases and will be central to the discussions in Paris, where President Xi Jinping met US President Barack Obama on the sidelines of the summit.

"As the two largest carbon emitters, we have both determined that it is our responsibility to take action," Obama said as he sat at a table with Xi.

But Xi also told the summit that that poor nations should not have to sacrifice economic growth.

"Addressing climate change should not deny the legitimate needs of developing countries to reduce poverty and improve their people's living standards," Xi said.

A Chinese government report released shortly before the summit raised concern at rising sea levels caused by climate change, which could threaten the country's developed east coast.

Beijing's "Third National Climate Change Assessment Report" said the sea levels have risen 2.9 millimetres (0.11 inch) annually from 1980 to 2012, according to the official China Climate Change website last week, while glaciers had shrunk over 10 percent since the 1970s.

Temperatures could rise by as much as five degrees Celsius (41 Fahrenheit) by the end of this century, the report added. Experts say this could fuel more dramatic sea level rises threatening China's most populated regions.

- Delhi smog -

Smog also blanketed the Indian capital New Delhi on Monday with PM2.5 levels soaring to 317.

India is the world's fourth largest emitter of greenhouse gases, and its coal use is rising rapidly alongside economic growth.

Delhi's air quality drops dramatically during winter when thousands of poor residents light fires to keep warm, and cloud cover traps the pollution from diesel trucks.

Authorities in the city have not issued any public health notices and thousands of runners took part in Delhi's half marathon on Sunday.

"A few days of breathing in the refreshing Delhi smog and suddenly I don't miss my daily cigarettes," resident Rahul Dutta Gupta said on Twitter.

In China PM2.5 levels in several cities in Hebei province bordering Beijing were also well above 500, official figures showed.

Motorways were forced to close in nearby Shandong province where visibility fell to less than 200 metres, the official Xinhua news agency said.

Beijing issued an orange-level pollution alert this weekend, its highest of the year, with residents advised to stay indoors.

Air pollution has been linked to hundreds of thousands of premature deaths, becoming a major source of popular discontent with the government.

- 'Airpocalypse' -

Beijing's severe pollution follows record-breaking smog in northeastern China last month, when PM2.5 levels reached 1,400 micrograms per cubic metre in the city of Shenyang -- the highest ever registered.

Intense pollution -- dubbed the "airpocalypse" -- also garroted the capital in 2013 when readings approached 1,000 micrograms per cubic metre.

Such outbreaks are common across China, where Greenpeace recently found nearly 80 percent of cities to have had pollution levels that "greatly exceeded" national standards over the first nine months of this year.

Xi is in Paris for the first day of the UN Conference of Parties (COP21) summit, which aims to strike a global deal limiting dangerous climate change.

China is estimated to have released between nine and 10 billion tonnes of carbon dioxide in 2013, nearly twice as much as the United States and around two and a half times the European Union.

It pledged last year that carbon dioxide output would peak by "around 2030" -- suggesting at least another decade of growing emissions.

Most emissions come from coal burning which spikes in winter along with demand for heating, which also causes smog.

Beijing's severe pollution is expected to last until a cold front arrives Tuesday, the city's environmental protection bureau said on its website.

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