MOSCOW (Reuters) - Russian President Vladimir Putin received former U.S. Secretary of State Henry Kissinger in the Kremlin on Thursday, the Kremlin said in a statement.

FILE PHOTO - Former U.S. Secretary of State Henry Kissinger speaks at the Nobel Peace Prize Forum in Oslo, Norway December 11, 2016. NTB Scanpix/Terje Bendiksby/via REUTERS

It did not elaborate on what the two men discussed, saying only that Kissinger was in Russia for the Primakov Readings, an annual forum of experts, diplomats and politicians.

Kissinger met U.S. President Donald Trump last month, and Putin and Trump are expected to meet on the sidelines of the Group of Twenty summit in Germany next week, in what would be their first face-to-face meeting since Trump became president.

Contacts between Russia and the United States are under intense scrutiny following allegations by U.S. intelligence officials of Russian interference in last year’s U.S. presidential election.

Russian officials, speaking on condition of anonymity, say they are frustrated that the Trump administration has not been able to engage effectively with Russia on important issues.

They say the issue is not Trump himself, but the stand-off inside the U.S. political system which, they say, means the U.S. president is liable to be attacked by domestic opponents if he makes any overtures towards Russia.

The officials say the amount of substantial contact Moscow has with the U.S. administration is minimal.

Illustrating the sensitivities around high-level contacts, the Kremlin acknowledged on Thursday that no formal bilateral meeting between Trump and Putin was planned for the G20 summit in Germany early next month, though the two were likely to cross paths.

Moscow is frustrated, according to the Russian officials, because it believes dialogue with the United States is crucial to resolving the conflict in Syria and concluding a peace deal which would allow Russia to end its military engagement there.

Kissinger, as national security adviser to U.S. President Richard Nixon, established a private channel of communication with Anatoly Dobrynin, the Soviet ambassador to Washington, which historians credit with helping ease Cold War tensions.