The Iranian-made drones that Russia sent on Monday to divebomb Ukraine’s capital delivered the most emphatic proof yet that Tehran has become a rare, increasingly close ally to the Kremlin, offering both weapons and international support that Russia sorely lacks.

There is no deep love between Russia, newly a pariah for attacking another country, and Iran, for decades one of the most strategically isolated nations in the world. But the two authoritarian governments, both chafing under Western sanctions, share a view of the United States as their great enemy and a threat to their grip on power.

“This is a partnership of convenience between two embattled dictatorships,” said Karim Sadjadpour, an Iran expert at the Washington-based Carnegie Endowment for International Peace.

Both countries are deep in crisis, struggling economically and politically. Iran is attempting to quell street protests that pose the most serious challenge in years to the government, while Russia is trying to manage rising dissension over a faltering war effort and an unpopular draft.

The emergence of a Moscow-Tehran alliance has multiple international implications, potentially dimming prospects for a new agreement to rein in Iran’s nuclear program and raising the pressure on Israel, Iran’s sworn enemy, to take Ukraine’s side in the war.

The relationship between Russia and Iran has been developing for years. President Vladimir V. Putin of Russia deployed his air force to Syria starting in 2015 to prevent the collapse of the regime of President Bashar al-Assad, a longtime ally of Tehran. Russia and Iran worked in lock step militarily, with Russian warplanes providing cover for Iranian militiamen and Iranian proxy forces fighting on the ground.

Syria was one example of the effort by both to find ways to sap American strength and prestige wherever they could in the world, and Ukraine provides a similar opportunity on an even larger, more visible scale.

After its 1979 revolution, Iran formulated foreign policy around the slogan “Neither East nor West,” equally wary of the Soviet Union and the United States. Now, the Islamic Republic is choosing sides, analysts said, and images of Iran’s exploding drones accurately hitting their targets advertise it as a regional power to be taken seriously.

In Tehran, the spokesman for the Iranian foreign ministry denied on Monday that his country was selling weapons to Russia, even as social media outlets linked to the Islamic Revolutionary Guard Corps, which developed the lumbering yet lethal drones, boasted about them.