On Monday, the U.S. accused Russia of aggravating political tensions in Venezuela. That prompted the Kremlin to repeat its claim that the White House has been trying to orchestrate a "coup."
It comes less than 48 hours after two Russian military planes touched down in Caracas, fueling speculation that Moscow is seeking to bolster its presence in the oil-rich, but cash-poor, country.
U.S. Secretary of State Mike Pompeo discussed the escalating crisis in Venezuela with his Russian counterpart Sergei Lavrov via telephone on Monday.
"(The U.S.) will not stand idly by as Russia exacerbates tensions in Venezuela," Pompeo said, according to a statement released by the State Department on Monday.
"The continued insertion of Russian military personnel to support the illegitimate regime of Nicolas Maduro in Venezuela risks prolonging the suffering of the Venezuelan people," Pompeo said, criticizing Moscow's "unconstructive behavior."
In response, Russia's Lavrov said: "Washington's attempts to stage a coup in Venezuela and threats against its legitimate government were a violation of the UN Charter and blatant interference in the internal affairs of a sovereign state," according to a statement by the foreign ministry on Monday.
Russia has long been an ally of Venezuela — lending the Latin American country billions of dollars and backing its oil industry and military.
Moscow and Caracas have sought to improve diplomatic relations in recent months, at a time when ties between the U.S. and Venezuela have dramatically deteriorated.
On Saturday, Javier Mayorca, a Venezuelan journalist, said on Twitter that he had seen a Russian air force Antonov AN-124 cargo plane and a smaller jet land near the capital city.
Images on social media also appeared to show Russian troops gathered on the tarmac of the public airport.
The Russian planes over the weekend are not the first to arrive in Venezuela in recent months.
In December, two Russian planes touched down in Venezuela, again triggering a war of words been Washington and Moscow. Russia and Venezuela have also held joint military exercises in recent weeks.
The U.S. has sought to ratchet up the pressure on Russia to sever its diplomatic, economic and military ties with Venezuela, but to no avail.
President Donald Trump has consistently refused to rule out the prospect of military intervention in Venezuela and the country's opposition leader, Juan Guaido, has previously called on the international community to "keep all options open."
Pressure is building on Maduro to step down.
More than 50 countries, including the U.S. and most Latin American and European countries, have recognized Guaido as Venezuela's legitimate leader.
It has thrust Venezuela into uncharted territory — whereby it now has an internationally-recognized government, with no control over state functions, running parallel to Maduro's regime.
The socialist leader has overseen a long economic meltdown, marked by hyperinflation, mounting U.S. sanctions and collapsing oil production.
As a result, some 3 million Venezuelans have fled abroad over the past five years to escape worsening living conditions.