MEXICO CITY (AFP) - Mexican President Enrique Pena Nieto has shortened an upcoming trip to major summits in China and Australia amid a growing human rights scandal over the disappearance of 43 college students.

Pena Nieto has cut four days from his trip, which will now run from Nov 9-15, according to a revised presidential schedule published Thursday by the Senate.

Pena Nieto will participate in the Asia-Pacific Economic Cooperation (Apec) summit on Nov 10-11 in Beijing before a state visit in China in his latest effort to forge closer ties with the Asian power.

He will then travel to Brisbane, Australia for the Group of 20 summit but will only attend the first day of the two-day meeting.

Officials did not say why the trip was shortened.

But Pena Nieto is facing the biggest crisis of his presidency after gang-linked police attacked busloads of students in the southern city of Iguala on Sept 26, leaving six people dead and 43 missing.

Tens of thousands of people protested Wednesday to demand the safe return of the students amid fears they were killed.

More than 200 students protested in front of the attorney-general's office on Thursday.

Jose Miguel Vivanco, the Americas director of Human Rights Watch, said the Iguala police attack was "one of the gravest cases recorded in the contemporary history of Mexico and Latin America".

"The human rights situation in Mexico is critical," Vivanco told a news conference, adding that another case, the alleged extrajudicial execution of at least eight gang suspects by soldiers in June, constituted a "crime of state."

Pena Nieto, he said, has kept the same posture regarding human rights as his predecessor, Felipe Calderon, who launched a controversial militarised war against drugs in 2006.

"Impunity is the norm in Mexico, and the Iguala case is extremely serious. It is a sign of the deep human rights crisis in Mexico," he said.