Chinese police in the southwestern province of Guangxi have shot dead one ethnic Uighur and detained a group of others who were trying to cross the border to neighbouring Vietnam.

The World Uighur Congress (WUC), an activist group in exile, told Al Jazeera on Wednesday that 43 Uighurs, including 19 children, were arrested on Sunday night amid a crackdown on members of the Muslim minority group who have increasingly sought better lives abroad.

The Chinese authorities, however, said only 21 people were detained, describing them as a group of "religious extremists".

Beijing has warned that religious extremists have been attempting to leave China to train with armed groups.

Officials said one Uighur was shot and killed by police, after he jumped out of the bushes and stabbed a policeman while others in the group were being handcuffed.

The WUC said the recent arrests were part of a deadly crackdown on Uighur refugees, who come mainly from the western Chinese province of Xinjiang.

Seyit Tumturk, the vice president of the WUC's Turkey and Middle East branch, said that "just a couple of months ago about four or five Uighurs were killed while trying to cross into Vietnam".

He also said that he feared for 300 Uighurs refugees in Thailand after Bangkok and Beiing signed a recent deal to step up cooperation in the fight against illegal immigration and terrorism.

Tumturk said that the agreement could lead to Thailand returning the group of Uighurs to China, which he described as a "death sentence".

The Chinese government blames Uighur separatist groups for unrest that has claimed hundreds of lives in recent years in and outside Xinjiang.

But international human rights groups say that Beijing's policies against the Uighur's culture and religion has resulted in systematic oppression, the imprisonment of many peaceful activists, and has forced more people to flee China.