Green algae about the size of a football pitch was seen 12 nautical miles off Qingdao, a port city in east China's Shandong Province, local maritime police said Saturday.

The large expanse of green algae, or enteromorpha prolifera, was first detected on May 28 off the east coast and expanded from 175 square kilometers on Thursday to 180 square kilometers on Friday, according to the North China Sea Branch (NCSB) of the State Oceanic Administration.

A local maritime police patrol vessel found scattered green algae around Dagong Island, seven nautical miles off the coastline, and saw some football-field-sized green algae five nautical miles away from Dagong Island on Friday, maritime policeman Bian Xiangfu said.

The green algae will continue moving toward China's east coastlines, especially Shandong Province, and might reach the waters off Qingdao in five days, according to the NCSB.

Although the green algae is not poisonous, it can consume large amounts of oxygen that can threaten marine life. Also, rotten algae emits a foul smell that can hinder tourism along the affected coastlines.

Green algae emerged in the Yellow Sea in 2007. Local residents and soldiers removed tens of thousands of tonnes of it in 2008 before the sailing events of the Olympics in Qingdao.