Taiwan received two upgraded early warning aircraft from the United States Sunday, the island's airforce said, as part of an arms deal that upset China-US relations.

The arrival of the E-2K aircraft in southern Kaohsiung city brings to four the number of such planes in Taiwan's air force, it said, after they were sent to the United States last year for upgrades.

Taiwan acquired four E-2Ts in 1995 and added two E-2K, an improved version of the Grumman Hawkeye series, in 2006.

A pair of E-2Ts were transported to the United States for upgrading last month, as 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.

Analysts say the E-2K -- an all-weather early warning and control system platform with an upgraded radar, software, avionics and propellers -- will further reduce warning time if China launched an air attack on the island.

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 Taiwan as part of its territory awaiting reunification, by force if necessary, even though the island 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.