Three international airlines have cancelled flights departing from Venezuela after a "potentially hazardous situation" warning.
The US Federal Aviation Administration (FAA) on Saturday issued an alert to airlines flying over the country, citing the "worsening security situation and heightened military activity in or around Venezuela".
The US is poised to launch a new phase of operations related to the South American nation in the coming days, four US officials said.
The exact timing or scope of the operations, as well as whether President Donald Trump has made the final decision to act, have not yet been established.
Brazil's Gol, Colombia's Avianca and TAP Air Portugal cancelled their flights from the capital, Caracas, in north Venezuela, on Saturday, according to Flightradar24 and the official website of Simon Bolivar Maiquetia International Airport.
TAP Air Portugal confirmed it cancelled flights scheduled for Saturday and next Tuesday.
"This decision follows information issued by the United States aviation authorities, which indicates that safety conditions in Venezuelan airspace are not guaranteed," the airline said.
The Trump administration has been considering Venezuela-related options to tackle what it has portrayed as President Nicolas Maduro's role in supplying illegal drugs that have killed Americans.
Venezuela's president has denied having any links to the illegal drug trade.
Mr Maduro, who is celebrating his 63rd birthday on Sunday and has been in power since 2013, has claimed Mr Trump is looking to oust him and that Venezuelan citizens and the military will rally against any such attempt.
Aeronautica Civil de Colombia said in a statement there were "potential risks" of flying in the Maiquetia area "due to the deterioration of security conditions and increased military activity in the region".
Spain's Iberia also said it was cancelling its flights to Caracas from Monday until further notice.
Reports of looming action have been growing in recent weeks as the US military has deployed forces to the Caribbean, amid heightened tensions with Venezuela. The Pentagon referred questions to the White House and the CIA declined to comment.
There has been a huge US military build-up in the region, including the US Navy's largest aircraft carrier, USS Gerald R Ford, at least eight other warships, and F-35 aircraft.
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The build-up in the Caribbean has been rumbling on for months, and Mr Trump has authorised covert CIA operations in Venezuela.
The US plans on Monday to designate the Cartel de los Soles, a Venezuela-based alleged criminal group, a foreign terrorist organisation for what the US sees as its role in importing illegal drugs into America, officials said.
The Trump administration has accused Mr Maduro of leading the cartel, which he denies.
US forces in the region so far have concentrated on counter-narcotics operations.
US troops have carried out at least 21 strikes on alleged drug boats in the Caribbean and Pacific since September, killing at least 83 people.
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