MEXICO CITY, Sept 16 (Reuters) - Two powerful storms pummeled Mexico as they converged from the Pacific and the Gulf on Monday, killing 34 people and forcing the evacuation of tens of thousands amid some of the worst flooding in decades.

Tropical Storm Ingrid battered Mexico's northern Gulf coast, while the remnants of Tropical Storm Manuel lashed the Pacific coast, inundating the popular beach resort of Acapulco, the U.S. National Hurricane Center said.

The storms unleashed torrential rains and killed nearly three dozen people in the states of Veracruz, Guerrero, Oaxaca, Puebla and Hidalgo, said Luis Felipe Puente, national emergency services coordinator.

State oil monopoly Pemex said it had evacuated three oil platforms and halted drilling at some wells on land due to the storms, but said output had not been affected.

"The storms have affected two-thirds of the entire national territory," the country's interior minister, Miguel Osorio Chong, said at a news conference in Mexico City.

Chong called the flooding "historic" and said the city of Acapulco had sustained major damage. Acapulco's international airport was closed temporarily due to power failure, as was a major highway, in the wake of Manuel.

In Veracruz state, along Mexico's Gulf coast, 12 people died on Monday after their bus was buried by a mountain landslide near the town of Xaltepec, Governor Javier Duarte told reporters.

Across the state, 23,000 people were evacuated from their houses and 9,000 remained in emergency shelters, according to a post on Duarte's Twitter account.

Public school classes in Veracruz were canceled for Tuesday.

Ingrid, which weakened from a hurricane earlier on Monday, prompted Pemex to evacuate three platforms at its offshore Arenque field, operated by British oil services firm Petrofac , and close 24 wells in its onshore Ebano-Panuco field, a company official said.

On Pemex's Twitter page, the company said it had activated "emergency procedures" at its Francisco Madero refinery on the Gulf coast of northern Tamaulipas state, but did not provide details. The Francisco Madero refinery has a processing capacity of 180,000 barrels per day, including crude from both the Arenque and Panuco fields.

Ingrid maintained sustaining maximum winds of 45 miles per hour (75 kph) and was expected to weaken as it moved overland.

The center said Ingrid was expected to become a tropical depression later on Monday while heavy rains continued as the storm churned 8 miles per hour (13 kph) toward the west-northwest. A tropical storm warming was in effect for La Cruz north to Rio San Fernando.

Manuel's maximum sustained winds stood at 30 mph (45 kph) as it dissipated over west-central Mexico, although heavy rainfall is expected to continue along the country's southwestern coast.

In Guerrero state, as many as 15 people died in landslides and as buildings collapsed after sustained weekend rainfall.

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