Taiwan has sent two early warning aircraft for upgrading in the United States and will send two more, media and the air force said Tuesday, as part of an arms deal that upset US ties with Beijing.

The island's air force confirmed that two US-made E-2T aircraft -- which have served the island's air force for 15 years -- are being upgraded in the United States and are scheduled to be sent back before the end of the year.

Another two of the planes were transported to the southern Kaohsiung harbour early in the morning under tight security, the state Central News Agency said. The air force declined to comment on that report.

Analysts say all four E-2Ts will be upgraded to the Hawkeye 2000 configuration, further reducing warning time if the Chinese were to launch an air attack on the island.

The upgrading was part of a $6.5 billion arms sale agreed with Taiwan in 2008, which also included advanced interceptor Patriot missiles, Apache attack helicopters and submarine-launched missiles.

Ties between Taipei and Beijing have improved markedly since Ma Ying-jeou of the China-friendly Kuomintang party came to power in 2008 promising to boost trade links and allow more Chinese tourists to visit the island.

But Beijing still sees the island as part of its territory awaiting reunification, by force if necessary, even though Taiwan has governed itself since 1949 at the end of a civil war.

China has repeatedly threatened to invade Taiwan should the island declare formal independence, prompting Taipei to seek more advanced weapons, largely from the United States.