Chinese maritime authorities Thursday added two large sea surveillance ships to its fleet in a bid to better protect the country's maritime rights and interests.

The two patrol ships, in the 1,000- and 1,500-tonne classes, respectively, were added to the North Sea fleet of the China Maritime Surveillance Force in the eastern coastal city of Qingdao.

They will be used to crack down on violations of China's maritime interests, illegal use of Chinese seawaters and damages to its sea environment, resources and infrastructures, said Fang Jianmeng, head of the North Sea branch of the State Oceanic Administration

The ships will also patrol China's waters to monitor polluting incidents, said Fang.

This is part of a 1.6-billion-yuan (241-million U.S. dollar) plan the State Council, or China's cabinet, unveiled in 1999 to add 13 1,000-tonne-plus sea patrol ships and five patrol helicopters to patrol the nation's waters.

The first group of six large patrol ships and two helicopters joined the China Maritime Surveillance Force under the State Oceanic Administration in November 2005.

A senior official of the China Maritime Surveillance Force, who declined to give his full name, told Xinhua that the agency has finished building the second group of three patrol ships and has purchased three helicopters.

"The remaining four vessels will be put into use before June this year," said the official, surnamed Wu.

The fleet expansion came as China is facing an increasingly heavier burden of safeguarding its seas rights and interests, said Wu.

China's Ocean Development Report 2010 released last May said the country's maritime rights and interests faced complicated situations and safety threats.

These include sovereignty over islands, sea delimitation, sea resources disputes, protecting the sea environment and new challenges such as delimitation of the continental shelf, safe passage on the seas and terrorism, it stated.

China has a coastline of 32,000 km and 350,000 square km of territorial seawaters and internal waters. It also has 3 million square km of its exclusive economic zone as recognized under the United Nations Convention on the Law of the Sea.

"Given the large sea territory, China's maritime surveillance force remains weak, even after all 13 patrol ships join the fleet," said Wu. "They're far from meeting all of our demands."

Even following the expansion, the fleet would have only 47 patrol ships, with 26 in the 1,000-tonne-plus class, Wu added.

Apart from the three fleets under the China Maritime Surveillance Force that cover the Bohai Sea, the Yellow Sea, the East Sea and the South Sea, the coastal provinces and municipalities also have their own regional sea patrol forces.

The regional forces planned to start building 36 sea patrol vessels this year to expand the county's sea surveillance fleet, Wu added.

The expansion is among the key measures that help protect China's maritime interests and promote a sustainable ocean economy, said Zhang Hongsheng, deputy director of the State Oceanic Administration.