US president Joe Biden has vowed to assist Ukraine in acquiring air "defense" systems that hold a longer range than the Stinger missiles already present. However, finding the powerful anti-missile batteries the Ukrainians have been pleading for, through their President, is "proving easier said than done."

Washington has been cautious about engaging in direct conflict with Russia. While Zelensky made a brazen request to establish a no-fly zone, the US asserted that it is "not interested" as it would immediately mean war. Meanwhile, Kiev seeks military assistance against Russian artillery. 

Read more: No no-fly zone? Send us aircraft, defense systems: Zelensky to US

Although it is argued that the ideal armament would be the mobile anti-aircraft batteries of the Patriot system, it is still unclear whether the Patriot system would be effective against the latest Russian missiles. 

The Ukrainian army, furthermore, is not trained to handle the Patriot system - however, they do know how to handle the S-300 anti-aircraft system, which is the Patriot's rival. Those may be provided by Soviet-bloc countries, like Slovakia and Bulgaria, but they will be demanding a Patriot substitute before handing them to Ukraine. 

"We're willing to do so immediately when we have a proper replacement," Slovak Defense Minister Jaroslav Nad said on Thursday.

Before Russia's military operation on February 24, the Ukrainian army had around 100 S-300 batteries, 40 of which are claimed to be destroyed by the Russian army, according to Brent Eastwood, a former US soldier.

"One S-300 is better than none," said Eastwood, explaining that multiple countries express readiness to provide S-300 missiles to Ukraine. However, Kiev is in need of many systems, including radars and monitoring stations.

"If I were a war planner for Ukraine, I would want four S-300 batteries for each quadrant of Kiev. This deployment would make me sleep better at night," he said.

Greece, to lend its S-300s, will need replacements - however, even if the Pentagon took a positive decision to provide Athens with Patriots in exchange for giving their S-300s to Ukraine, it would take weeks for the Patriots to reach Greece.

However, Greece made it clear, through its chief of the Hellenic National Defense General Staff Konstantinos Floros, that it will not be sending S-300 systems since this will weaken its defense capabilities.

"Obviously, we do not consider either relocation or transfer of weapons, it would weaken the country's defense capabilities," Floros told reporters. 

Read more: Greece not to transfer of S-300 systems to Ukraine

On Wednesday, CNN reported that the United States and its NATO allies are sending surface-to-air missile systems to Ukraine, including Soviet-era systems, such as the SA-8 and the SA-12 systems, citing a senior US official.

The systems have a higher engagement altitude than the Stinger shoulder-held missiles the US had sent to Kiev.

The systems are "on the way" to Ukraine, the official added.