The reports have put new focus on how some Chinese migrants appear to view Southeast Asia as a conduit to a new life overseas.

Five Chinese nationals and two Vietnamese border guards died in the initial confrontation on Friday. The guards had detained 16 people and were preparing to return them to Chinese authorities when several of the migrants seized firearms from the Vietnamese security forces and began firing, according to Vietnamese state media. Some of the migrants died after leaping from the multistory building where they were being held, the reports said.

The second group of 21 migrants was intercepted at sea after Vietnamese border guards were tipped off by Chinese authorities, state-controlled VTC News reported. Vietnamese foreign ministry officials didn't respond to calls seeking comment. China's foreign ministry confirmed the earlier, violent incident on its microblogging account.

It is unclear who comprised either group of migrants or where they were intending to travel. Some of the women involved in the earlier fracas wore scarves across their faces in a manner reminiscent of the Uighur ethnic group from the Xinjiang region in western China.

Growing numbers of Uighurs have tried to escape China through Vietnam and Laos in recent years because of what human-rights organizations describe as religious and political persecution.

Thai authorities last month detained several groups of migrants, the largest of which consisted of 220 people, many of whom said they were from Turkey and had entered the country illegally. Uighurs, who are mostly Muslim, speak a Turkic language. Thai authorities said they believed that the groups were aiming to make their way to another country.

New York-based Human Rights Watch, responding to reports that Chinese diplomats had identified dozens of the migrants as being Uighurs, urged Thailand not to send them back to China.

"Past cases have shown that Uighurs returned to China are always at risk of persecution," said Brad Adams, Human Rights Watch's Asia director.

Conflicts between China's Uighurs and the majority Han Chinese population have escalated in Xinjiang in recent years. The region borders Pakistan, Afghanistan and parts of former Soviet-controlled Central Asia. Ethnic Uighurs complain that their religion and culture are being suppressed by an influx of Han migrants, while Beijing accuses separatists in Xinjiang of using terrorist tactics to further their cause.

Interethnic riots in the regional capital Urumqi in 2009 left around 200 people dead and drew global attention to the resentment many Uighurs feel toward Chinese rule. China responded by trying to channel economic growth to the area by opening up more trade links with Central Asia. Some Uighurs, though, complained that the benefits of the program were concentrated in the hands of state-owned enterprises and ethnic-Han entrepreneurs who have migrated to the region.

A mass stabbing at a train station in Kunming, southwest China, killed 33 people in early March. Chinese state media said at least 10 assailants were involved in the attack, the deadliest ever attributed to Xinjiang separatists outside their home territory.

The Chinese authorities' subsequent crackdown has further raised tensions in Xinjiang.

By James Hookway & Nguyen Pham Muoi - The Wall Street Journal - April 20, 2014