Secretary of State Mike Pompeo told his Russian counterpart Monday that the United States “will not stand idly by” if Russia continues to send military personnel to Venezuela to prop up President Nicolás Maduro, the State Department said.

Pompeo’s telephone call with Russian Foreign Minister Sergei Lavrov followed weekend reports that at least two Russian air force planes carrying a senior defense official and about 100 troops landed at Venezuela’s main airport Saturday.

The United States and dozens of other countries in January recognized Juan Guaidó, the head of Venezuela’s opposition-controlled National Assembly, as interim president, but Maduro has managed to hold on to power.

U.S. frustration with the situation has grown as efforts to persuade the Venezuelan military to switch sides and support Guaidó have produced few results, even as thousands of anti-Maduro protesters have taken to the streets amid severe shortages of food, water and medical care.

The military and pro-Maduro paramilitary groups have escalated the use of force against the Guaidó supporters.

The Trump administration has accused Russia and Cuba, Maduro’s main backers, of intervening to keep him in power.

“The continued insertion of Russian military personnel to support” Maduro “risks prolonging the suffering of the Venezuelan people who overwhelmingly support” Guaidó, the U.S. statement said. Pompeo, it said, called on Russia, to “cease its unconstructive behavior and join other nations . . . who seek a better future for the Venezuelan people.”

Russia’s Foreign Ministry said that the call took place on Pompeo’s initiative and that the secretary was interested in “specific issues” related to Venezuela and Syria.

The Russian statement did not comment directly on reports of the military landings, but said Lavrov “emphasized that Washington’s attempts to organize a coup d’etat in Venezuela and threats against its lawful government constitute a violation of the U.N. Charter and open interference in the domestic affairs of a sovereign state.”

Separately, Nikolai Patrushev, head of the state security council, accused the Trump administration of acting to “promote U.S. economic interests” and “take over Venezuelan oil,” according to an interview with the Izvestia newspaper published Monday. Last month, Patrushev charged that the United States was deploying troops to Colombia and Puerto Rico ahead of a planned military intervention to topple Maduro.

President Trump has repeatedly said that “all options,” including military intervention, are on the table in dealing with Venezuela. But administration officials have said that there are no current plans to use military force and that U.S. policy is focused on diplomatic and economic pressure through escalating sanctions. Earlier this month, the administration evacuated U.S. diplomatic personnel from Venezuela.

There have been a number of unconfirmed reports in recent weeks of Russian military and contract soldiers appearing in Venezuela. On Saturday, a local reporter in Caracas, Javier Mayorca, wrote on Twitter that a plane carrying about 100 troops and Vasily Tonkoshkurov, chief of staff of the Russian ground forces, landed in Caracas. It was followed by a cargo plane carrying 35 tons of materiel, he wrote.

Flight tracking websites said the planes had taken off from Russian military airports. Russia made no official comment on the flights, although the government-owned Sputnik news agency on Monday quoted an unnamed “diplomatic source in Caracas” as saying Russian military personnel had arrived “to take part in bilateral consultations” with the Maduro government.

In December, Russia sent two nuclear-capable long-range bombers to Venezuela for several days to participate in what it said were joint exercises.

Russia and China have blocked U.S. proposals in the United Nations for international sanctions against Venezuela.

Since the election of Hugo Chávez as Venezuela’s president in 1999, Russia has been the country’s primary military supplier. Under Maduro, who became president after Chavez’s death in 2013, Moscow has extended billions in loans to Caracas, much of it under arrangements to repay the debt with Venezuelan oil.

Rosneft, Russia’s government-owned oil behemoth, has provided Maduro with much-needed cash for joint ownership of at least five Venezuelan oil fields.

Venezuela has the largest known petroleum reserves in the world, but production has plummeted in recent years because of corruption and mismanagement. The current political and economic crisis has sent millions of Venezuelans fleeing into other countries in the hemisphere.

Cuba, with economic problems of its own, has sent what the administration says is up to 25,000 military and intelligence personnel — and Havana says are medical and educational personnel — to Venezuela in exchange for free and subsidized oil.