BEIJING: Tangshan, China's top steelmaking city, has extended recent emergency pollution control measures until July 21 because of a possible major smog alert, according to a document seen by Reuters and a source familiar with the matter.The measures, which include the closure of some sintering plants and output curbs on coke and cement factories, came into force on July 13 and were due to last until noon on Wednesday. Sintering plants that comply with emissions requirements are exempt from the controls.Forecast smog in Tangshan, in China's northern Hebei province surrounding Beijing, may trigger an "orange" pollution alert, the second-highest in China's four-tier warning system after red, according to the document. The Tangshan government could not immediately be reached for comment.China's cabinet last week launched a new cross-ministerial leadership group that will help draw up plans to tackle pollution in smog-prone northern regions around Beijing.