Authorities in eastern Mexico state Veracruz have ordered the evacuation of Tlacotalpan, a United Nations heritage site threatened by flooding of the nearby Papaloapan River.

Statewide, some 180 small towns spread across 75 municipalities were flooded over last weekend, while some 10,000 residents have already moved into emergency shelters, local broadcasters reported on Monday.

Schools in 48 municipalities will remain closed until Friday.

Tlacotlapan, a port town in Veracruz, was named a heritage site by the United Nations Education, Science and Culture Organization (Unesco) in 1998 because its preservation of 16th century building structure and city street plan to "an exceptional degree," according to the statement issued by Unesco at the time.

The flooding was caused by the opening of floodgates at the Temascal and Cerro de Oro reservoirs, which had been overfilled by rainwater brought onto the Mexican mainland by Hurricane Frank. Frank did not come onshore in Mexico, but its rain bands triggered stronger across the south of the nation, which has travelled downstream to the east. Frank formed on Aug. 21 and dissipated last Friday.

The state-run Mexican Water Institute has called for the building of locks on the River Papaloapan to protect residents, a project first mooted in 1944, when the Cerro de Oro and Temascal reservoirs were built.