President Trump jabs at the Russian leader with threats; Vladimir Putin responds with flattery. But there are notable signals in their jousting, including a revived discussion about nuclear arms control.

Russia’s president flattered his American counterpart as “smart” and “pragmatic,” and repeated Mr. Trump’s baseless claim that the 2020 election was stolen.

President Vladimir V. Putin of Russia praised President Trump’s second inauguration, but the American leader did not return the compliment.

Russia’s president is said to have blamed birds or a Ukrainian drone for a Dec. 25 crash of an Azerbaijani plane. Azerbaijan says Russian air defenses were at fault.

In the crash’s aftermath, Azerbaijan has unleashed rare and stinging criticism of Russia, with the country’s president saying Moscow’s response has caused “surprise, regret and rightful indignation.”