By Julia Symmes Cobb and Alexandra Ulmer

BOGOTA/CARACAS, July 27 (Reuters) - Colombian airline Avianca said on Thursday it would suspend flights to and from Venezuela immediately, instead of in mid-August, sparking questions about what caused its accelerated retreat from the crisis-hit socialist nation.

The latest commercial carrier to pull out of Venezuela said in a statement it was obliged to cancel the flights because of "operational and security limitations registered during the last few hours."

Avianca Holdings SA had said on Wednesday it would cancel its Bogota-Caracas-Bogota and Lima-Caracas-Lima routes, a total of three daily flights, beginning Aug. 16.

Avianca's retreat further isolates Venezuela at a time when many airlines have left the country, citing everything from payment disputes to safety concerns.

Avianca's move comes against a backdrop of major anti-government unrest and a crippling economic crisis that has millions of Venezuelans struggling to eat properly.

But losing one of the most regular and easiest flights to a neighboring country particularly shocked Venezuelans. Many were wondering what sparked Avianca's decision to end operations earlier than announced.

A Venezuelan opposition lawmaker from Vargas state, where the airport serving Caracas, the capital, is located, said Avianca had suspended its operations early because of threats from President Nicolas Maduro's government.

"Maduro threatened Avianca with seizing their plane and never giving them flight permission. Plus he banned use of Venezuelan air space!" tweeted lawmaker Jose Manuel Olivares.

Flight data from Flightradar24 showed an afternoon flight between Bogota, the Colombian capital, and the Caribbean island of Aruba, which would normally fly over western Venezuela, avoiding Venezuelan airspace.

Avianca and Venezuela's Information Ministry did not immediately respond to a request for comment. A spokesperson at Venezuela's Maiquetia airport was not immediately available.

COLOMBIA-VENEZUELA TENSIONS

The South American neighbors have for years had a volatile relationship, with Venezuela's government sometimes accusing Colombia of provocation, and claims in the other direction that Caracas has supported Colombian guerrillas.

Many airlines have been in protracted disputes with the Venezuelan government over billions of dollars of ticket sales made in the beleaguered local currency.

They say Maduro's administration has failed to reimburse them amid strict currency controls.

The International Air Transport Association said last month Venezuela owes airlines more than $3.8 billion.

United canceled its Venezuelan flights in June. Lufthansa and Air Canada have also pulled out.

Avianca said its customers will be rebooked on other airlines still flying the route or be reimbursed in full. (Additional reporting by Girish Gupta in Caracas; Editing by Dan Grebler)