Slovakia has transferred an S-300 missile defense system to Ukraine, fulfilling one of Ukrainian President Volodymyr Zelensky's top requests to help the country defend itself against Russia's bombing campaign.

Why it matters: Zelensky pleaded in an address to the U.S. Congress last month for the U.S. and its European allies to impose a no-fly zone or give Ukraine the ability to "close the skies" itself by facilitating the transfer of Soviet-era fighter jets or anti-aircraft systems.

The latest: President Biden thanked the Slovakian government in a statement Friday and said the U.S. will reposition a Patriot missile system to Slovakia.

• The Patriot missile system will be manned by U.S. service members at the invitation of the Slovakian government.
• "This deployment of Patriot capabilities to Slovakia aligns perfectly with our previous efforts to bolster NATO’s defensive capabilities and to demonstrate our collective security requirements under Article 5 of the NATO treaty," Defense Secretary Lloyd Austin said in a statement.

What they're saying: "I would like to confirm that Slovakia has provided #Ukraine with an air-defence system S-300. Ukrainian nation is bravely defending its sovereign country and us too. It is our duty to help, not to stay put and be ignorant to the loss of human lives under Russia’s aggression," Slovakia's Prime Minister Eduard Heger tweeted.

The big picture: In addition to the S-300, Ukraine has received tanks and infantry fighting vehicles from the Czech Republic, as the Donbas region braced for a major Russian offensive.

• "As the Russian military repositions for the next phase of this war, I have directed my Administration to continue to spare no effort to identify and provide to the Ukrainian military the advanced weapons capabilities it needs to defend its country," Biden said in his statement Friday.
• U.K. Foreign Secretary Liz Truss told reporters Thursday that NATO countries had expressed support for providing "new and heavier equipment" to Ukraine as outrage builds over Russia's atrocities against civilians.
• "The battle for Donbas will remind you of the Second World War with large operations, thousands of tanks, armored vehicles, planes, artillery," Ukrainian Foreign Minister Dmytro Kuleba said at the NATO meeting Thursday, pleading for allies to accelerate their military aid.

Go deeper:

• Axios explains Ukraine
• The latest on the Russia-Ukraine crisis

Editor's note: This story has been updated with more details on the Patriot air defense system and comment from President Biden and Defense Secretary Lloyd Austin.