(Corrects date of China protest in paragraph 5)
* Japan PM sends offering to shrine just before Obama visit
* Japanese spokesman says Chinese court order to seize ship "regrettable"
* China protests over earlier shrine visits
* Court cases arise seeking compensation for forced wartime labour
By Elaine Lies
TOKYO, April 21 (Reuters) - Japanese Prime Minister Shinzo Abe has sent a ritual offering to the Yasukuni Shrine, seen by critics as a symbol of Japan's past militarism, a move that may increase strains on Tokyo's ties with Asian neighbours China and South Korea.
Adding to unease in the region, a Chinese maritime court in Shanghai seized a ship on Saturday owned by Japanese shipping firm Mitsui O.S.K. Lines, saying the company had failed to pay compensation stemming from a wartime contractual obligation.
The offering from Abe, who visited the shrine in December but opted not to go in person this time, was sent just before U.S. President Barack Obama's three-day visit to Japan begins on Wednesday.
The United States has said it was "disappointed" with Abe's shrine visit last year, which infuriated Beijing and Seoul.
China protested on April 12 after internal affairs minister Yoshitaka Shindo visited the shrine, where 14 Japanese leaders convicted as war criminals by an Allied tribunal after World War Two are honoured along with Japan's war dead.
Abe made his latest offering to the shrine as a private individual so it was not the government's place to comment, Chief Cabinet Secretary Yoshihide Suga told a news conference.
"It will not have an impact on the U.S.-Japan leaders meeting," Suga, the chief government spokesman, said on Monday.
Keiji Furuya, another cabinet minister, has also paid his respects and a number of lawmakers are expected to visit the shrine during its spring festival this week.
Abe has said that, like predecessors such as former premier Yasuhiro Nakasone who visited the shrine, he had high regard for Japan's ties with China and South Korea, which suffered under Japanese occupation and colonisation in the 20th century.
A number of court cases demanding compensation for forced wartime labour have arisen in China and South Korea. In February, two Japanese firms were sued in what media said at the time was the first instance where a Chinese court had accepted such a case.
Suga said the ship seizure, apparently the first time the assets of a Japanese company have been seized in a lawsuit concerning compensation for World War Two, was "extremely regrettable".
"It is inevitable that this will have an adverse impact on Japanese companies in China," he said. "We strongly urge the Chinese government to make the proper response."
A spokesman for Mitsui O.S.K. said the company has been informed of the seizure order but was still trying to assess what was happening at the port. It did not confirm that the vessel was in the hands of the court.
The ship, "Baosteel Emotion", is a 226,434 deadweight-tonne ore carrier.
One analyst said the impact of the seizure was likely to be limited and noted that it just seemed to be another case of China putting pressure on Japan, adding that it was unclear if this represented the policy of the Chinese leadership.
"Companies that are currently involved with such issues will likely think twice about Chinese businesses, but I believe most Japanese companies have nothing to do with these problems," said Akio Takahara, a professor at the University of Tokyo.
"They might take this development as one factor when they think about investments in China, but I don't think this is a decisive factor at the moment."
Obama's Asian trip will also take him to South Korea, Malaysia and the Philippines. (Reporting by Kiyoshi Takenaka, Tim Kelly, Dominic Lau and Edmund Klamann; Writing by Elaine Lies and Linda Sieg; Editing by Paul Tait)