* Tangshan's orange alert to remain in place until further notice
* City accounts for around 12 percent of China's steel output
* Sintering, coke plants among those affected (Adds detail, background)
BEIJING, March 1 (Reuters) - China's top steelmaking city of Tangshan on Thursday said it would extend a level 2 or "orange" pollution alert until further notice, leaving steel and coke plants under tighter restrictions.
Tangshan, located in China's northern Hebei province, had originally put the alert in place from Feb. 24-28 due to heavy smog.
An orange alert, the second-most severe in China's pollution warning system after red, is issued when forecast readings of particulate matter with a width of 2.5 microns, known as PM2.5, are above 200 microgams per cubic metres for three consecutive days, including one day above 300 micrograms.
The alert last week ordered Tangshan sintering plants to stop production for 48 hours and then operate for a maximum of 12 hours before stopping again. Coke plants must extend their production time to over 48 hours, meaning less dust will be produced during the process.
Factories in heavily polluted Tangshan have been forced to cut production by as much as 50 percent during China's 2017/18 winter months as part of a targeted environmental protection campaign.
The city, which accounts for about 12 percent of the country's steel output, said last month it would extend restrictions on production after the end of the winter heating season on March 15. It plans to ask mills to cut utilisation rates by 10-15 percent from March 16 to Nov. 14 to improve air quality.
Shanghai's most traded steel rebar contract was up 0.5 percent at 4,059 yuan ($639.91) a tonne at 0303 GMT, reaching its highest in nearly three months. ($1 = 6.3431 Chinese yuan renminbi) (Reporting by Tom Daly; editing by Richard Pullin and Joseph Radford)