China has dispatched two patrol ships to 'assert its sovereignty' over islands at the centre of a row with Japan, state media said on Tuesday, as Tokyo completed its purchase of the disputed territory.

The two marine surveillance ships had reached the waters around the Diaoyu islands - known in Japan as the Senkaku islands - and would 'take actions pending the development of the situation', the Xinhua news agency said.

The arrival came as the Japanese government announced it had completed its planned purchase of the islands, which lie in a strategically important shipping area with valuable mineral resources thought to be nearby.

"This should cause no problem for Japan's ties with other countries and regions," said Japan's Chief Cabinet Secretary Osamu Fujimura.

"We have absolutely no desire for any repercussions as far as Japan-China relations are concerned. It is important that we avoid misunderstanding and unforeseen problems," he told reporters.

Beijing had earlier summoned the Japanese ambassador and lodged a strong protest over Tokyo's move to purchase the islands, while vowing to take counter-measures.

Chinese Premier Wen Jiabao said the islands were 'an inherent part of China's territory' and vowed his country would 'never ever yield an inch' on its sovereignty.

However, the ships China dispatched were from the State Oceanic Authority and not military vessels and analysts downplayed the significance of the move, saying the deal may even allow Beijing and Tokyo to temper tensions.

"That some patrol vessels were deployed in the vicinity of the islands was almost inevitable, but now, at least, there is no longer a risk that some nationalist Japanese politician would gain control of the islands," said China expert Jonathan Holslag.

"Most decision-makers in Beijing are relieved that the Japanese national government bought the Senkaku/Diaoyu Islands," added Holslag, head of research at the Brussels Institute of Contemporary China Studies.

On Sunday, Chinese President Hu Jintao urged Japanese Prime Minister Yoshihiko Noda not to go ahead with the purchase in brief talks held on the sidelines of the Asia-Pacific economic summit in Vladivostok.

"China-Japan relations have recently faced a severe situation due to the Diaoyu Island issue," a foreign ministry statement quoted Hu as telling Noda.

"Japan must fully recognise the gravity of the situation and should not make wrong decisions."

Officials at China's State Oceanic Administration, which dispatched the two surveillance ships, were not immediately available to clarify to AFP whether the vessels were armed.

Often testy Japan-China ties took a turn for the worse in August when pro-Beijing activists landed on one of the islands.

They were arrested by Japanese authorities and deported. Days later about a dozen Japanese nationalists raised their country's flag on the same island, Uotsurijima, prompting protests in cities across China.

Japan's government currently leases four islands and owns a fifth. It does not allow people to visit and has a policy of not building anything there.

State television and all major Chinese dailies in China on Tuesday highlighted Beijing's condemnation of the purchase. Around 200 people in eastern Shandong province took to the streets on Tuesday to protest, carrying banners and singing China's national anthem.

The islands, which lie around 200 kilometres (125 miles) from Taiwan and 2,000 kilometres from Tokyo, are also claimed by Taipei, which strongly protested the Japanese move on Tuesday.

"We strongly demand that the Japanese government revokes this move," Taiwan's foreign minister Timothy Yang told reporters in Taipei.

"Japan's unilateral and illegal action cannot change the fact that the Republic of China (Taiwan's official name) owns the Diaoyu islands."