Environmental officials have warned Beijing it is highly unlikely that the city will hit its target of 238 "blue sky" days this year.

The capital needs at least 22 such days per month from June to December to achieve its aim.

"It is indeed very tough (to have over 22 'blue sky' days a month)," said Du Shaozhong, deputy director of the Beijing Municipal Bureau for Environmental Protection.

Beijing saw only 11 "blue sky" days, or days with fairly good air quality, April, the least in five years, Du said.

From January to April, Beijing reported only 51 "blue sky" days, 16 less than the corresponding period last year, he said.

Despite having already had 22 clear days by Monday this month, Du showed no optimism.

"We are still confronted with a severe situation in achieving the 'blue sky' goal, and we call for joint efforts of all citizens to reduce the emission of pollutants," he said while promoting a latest city-wide campaign of "no car day" to clean up air pollution.

More than 200,000 drivers from about 100 Beijing automobile associations have been encouraged to leave their cars at home at least one day a month in a hope to ease traffic jams, reduce noise and improve air quality in the Chinese capital.

Motor vehicle emission is the leading cause of Beijing's air pollution, Du said.

According to the city's environment department research, Beijing's vehicles spew out 3,600 tons of pollutants each day.

There are more than 2.6 million motor vehicles in Beijing and the number is increasing by more than 1,000 a day, Du said.

The "no car day" for next month was set on June 5, next Monday, he said.

Beijing launched the program called "Defending the Blue Sky" in 1998, when the city had only 100 days of blue sky.