A Chinese research ship and its escort, which operated for nearly a month in Vietnam's exclusive economic zone (EEZ) in the South China Sea, left those waters on Monday night, vessel-tracking experts said, just after high-level U.S.-China talks.

Chinese vessel Xiang Yang Hong 10 began sailing in Vietnam's EEZ on May 7, at times flanked by a dozen ships, and regularly crossing gas and oil fields operated by Russian companies, according to vessel-tracking data.

On Monday, after U.S. and China senior officials held talks in Beijing that both parties called constructive, the Chinese vessel and its escort of more than half a dozen ships began their journey back to China's Hainan island, leaving Vietnam's EEZ around midnight.

Asked about the movements of the ships, China's foreign ministry did not comment about the return to Hainan.

"The Chinese scientific research vessel carrying out normal research activities in maritime waters under China's jurisdiction is legitimate and proper. The issue of entering another country's EEZ does not exist," it said in a statement.

Under international law, ships are allowed to sail through foreign EEZs, but unauthorised surveys are not permitted and China's operations in the South China Sea have long been problematic for countries in the region, as Beijing claims most of the energy-rich sea, including foreign EEZs.

In a rare public protest, Vietnam's government on May 25 urged the Chinese research ship and its escort to leave the country's EEZ after a visit to Hanoi by senior Russian official Dmitry Medvedev.

At 0300 GMT on Tuesday the Chinese research ship was seen approaching Hainan, said Ray Powell, who leads Stanford University's Project Myoushu on the South China Sea.

Vietnam's fisheries surveillance ships turned back after the Chinese vessel and its escort left Vietnam's EEZ around midnight Vietnam time, Powell added.

Vietnam-based researcher Van Pham, who heads independent non-profit South China Sea Chronicle Initiative (SCSCI), confirmed the ships had left Vietnam's EEZ but warned that Hainan was not the research ship's home port, and after a break there it could resume activities in the South China Sea.

Vietnam's foreign ministry did not reply to requests for comment.