Russian Foreign Minister Sergei Lavrov Monday accused the United States of plotting a "coup" against Venezuelan President Nicolas Maduro, in a dispute over Russian military assistance for the crisis-hit country.

Lavrov said in a statement he had complained of "attempts by Washington to organise a coup d'etat in Venezuela" during a telephone conversation with US Secretary of State Mike Pompeo.

The statement said such moves "constitute violations of the UN charter and undisguised interference in the internal affairs of a sovereign state".

Pompeo earlier warned Russia that the United States will not "stand idly by" as Moscow inserts military personnel into Venezuela to support Maduro, a State Department statement said.

Russian state news agency Sputnik reported Sunday that two Russian military planes delivered troops and equipment to Venezuela over the weekend.

Lavrov did not comment on that report.

Pompeo said Russian military reinforcements were prolonging the political crisis in the South American country.

Venezuela is rich in oil but has plunged into economic crisis, suffering hyperinflation and shortages of food and basic goods.

Washington and its allies support Venezuelan opposition leader Juan Guaido and recognise his claim to be acting president in defiance of Maduro.

Socialist leader Maduro retains the support of Russia and China, US rivals who have offered him political and economic support. (AFP)