China is stamping return to sender on mail from Taiwan postmarked with a slogan supporting the island's bid to join the United Nations.

Taiwan's post office began putting a "U.N. for Taiwan" postmark on selected items of outgoing mail six weeks ago.

Letters and parcels bearing that slogan and one saying "Taiwan joining the United Nations" were being returned as a protest against alleged independence activities by the government of Taiwanese President Chen Shui-bian, according to Fan Liqing of China's Taiwan Affairs Council.

Liu Te-shun of Taiwan's Mainland Affairs Council — the Cabinet-level body in charge of implementing China policy — said China's action contravened international mail practice.

"It is common for countries to stamp commemorative slogans on mail," he said.

Since their split amid civil war nearly 60 years ago, Taiwan and China have confronted each other angrily across the 160-kilometer (100-mile) Taiwan Strait.

China regards the democratic island as part of its territory and has threatened to attack if it formalizes its de facto independence.

The rhetoric escalated this year when Chen's government started a campaign to win U.N. membership using the name Taiwan rather than its official title of the Republic of China.

The distinction is crucial because the Republic of China connotes continued support for the one-China concept that has been at the core of China's Taiwan policy since their split in 1949, while the Taiwan title reflects support for the pro-independence stance of Chen's Democratic Progressive Party.

China has expressed strong opposition not only to Chen's U.N. bid but also to earlier bids he has launched to emphasize Taiwan's separateness from the mainland.

Since scrapping a government body responsible for unification with the mainland early last year, Chen has systematically attacked the legacy of late dictator and unification icon Chiang Kai-shek, and stricken the China name from a number of government companies — replacing it with Taiwan.

He has also pushed for far-reaching changes to school textbooks to de-emphasize Taiwan's historical and cultural links to the mainland.