Towering waves whipped up by tropical storm Muifa breached sea defenses protecting a petrochemical plant near a northeastern coastal city on Monday, raising fears of a toxic spill, before the breaches were plugged with thousands of tons of rocks and concrete slabs.

Tropical storm Muifa breaches sea defenses protecting a petrochemical plant near Dalian city, Liaoning Province on August 8, 2011. [China.com]

At about 3:30 am, two sections of a dike, each at least 30 meters wide, were breached by waves up to 20 meters high at the Jinzhou Industrial Zone in Dalian city, Liaoning Province, according to local authorities.

The breaches threatened toxic chemical tanks at the petrochemical plant, located about 30 kilometers from downtown Dalian.

The nearest tanks were located about 50 meters away from the dike.

As of Monday evening, authorities had yet to confirm or deny if leaks had occurred in any of the tanks.

Don't miss: 1.35 mln people evacuated as Muifa nears

The Fujia chemical plant produces paraxylene, a petrochemical used to produce raw materials for the production of polyester film and fabrics.

Workers at the plant had already been evacuated before the breach as the tropical storm approached, Li Ming, a migrant worker from Anhui Province, said.

About 400 workers had been contracted to install petrochemical equipment at the plant.

Local authorities deployed about 400 heavy trucks to transport about 20,000 tons of rocks and concrete slabs to plug the breaches.

Troops and police also helped evacuate nearby residents, said a report on the People's Daily website, which gave no details of any damage or injuries.

By 3 pm, the situation was under control, local authorities said in a statement.

The plant transported toxic chemicals out of the tanks nearest the dike, a spokesman for the local government, said.

Trucks continued to dump rocks and concrete slabs to reinforce the dike on Monday afternoon.

After passing off the Liaoning coast, tropical storm Muifa, previously a typhoon, landed in northwestern areas of the Democratic People's Republic of Korea (DPRK) on Monday afternoon, said Yu Qinghua, an official at the flood prevention headquarters in Dandong city, Liaoning, which borders the DPRK.

The DPRK's official Korean Central News Agency said heavy rain was falling and expected throughout the night in northwestern provinces as the storm moved northward.

More than 490,000 people were evacuated in Liaoning and Shandong provinces on Monday.

The National Meteorological Center reported earlier coastal winds up to 75 km/h in Liaoning and Shandong provinces and heavy rain in Liaoning on Monday afternoon.

In Shandong, more than 20,000 fishing boats were called back to harbor, local authorities said.

There have been no reports of any casualties.

But the storm left four people dead and two others missing as it battered the Republic of Korea's (ROK) west coast on Monday.

Winds toppled hundreds of power lines, signposts and trees, and power was cut to 320,000 houses in southwestern provinces in the ROK.

But a storm warning was lifted for most of the country on Monday afternoon.

The southern resort island of Jeju was among areas hardest hit, with some places on the island hit by up to 600 millimeters of rain on Sunday and Monday.

Scores of flights were canceled.