Nearly three weeks after Russia’s invasion of Ukraine, the Biden administration is still determining the most effective way to get military aid into the country without sparking a larger conflict.

While administration officials claim that the effort to provide Ukrainian forces with American-made weapons like Javelin anti-tank and Stinger anti-aircraft missiles has been largely successful, the White House is under intense pressure to do more.

Congress and Ukrainian President Volodymyr Zelensky, in particular, have pressed Biden officials to assist in the transfer of heavier Soviet-made weaponry to Ukraine, such as MiG fighter jets and S-300 surface-to-air missile systems. It’s a message Zelensky may repeat when he addresses Congress on Wednesday.

However, so far, the Biden administration has not aided in the provision of those heavier, Soviet-made weapons systems. US officials are still worried about how Russia will react if certain weapons transfers are approved. Officials have also questioned whether MiG fighters would be useful and whether the risk is worth it.

According to sources, the State Department has been working to identify which countries currently have the Soviet-made S-300 air defense systems and is looking into how they could be transferred to Ukraine. According to multiple sources familiar with the discussions, it is still unclear how the US would supplement those countries’ supplies of surface-to-air missiles.

A similar issue arose last week, when Poland indicated its willingness to send MiG-29 fighter jets to Ukraine, and the US informed Poland that it would be unable to backfill the jets quickly.

According to multiple sources familiar with the status of the arms shipments, almost all of the $350 million in weapons approved by the Biden administration for Ukraine in late February have been delivered. According to two sources familiar with the situation, President Joe Biden approved an additional $200 million military aid package for Ukraine over the weekend, which includes more Javelin and Stinger missiles.

According to sources familiar with the situation, US officials have told Congress that the weapons they are sending to Ukraine are making their way there. However, according to two sources, the NATO alliance has no idea what percentage of military assistance is actually reaching Ukrainian forces after it crosses NATO and European territory.

NATO is unaware because these shipments are being arranged bilaterally – directly with Ukraine – rather than through the alliance, according to sources. Nations have been instructed not to discuss with one another what they are sending and how they are sending it.

This is partly due to security concerns, as Western officials do not want Russians to target the convoys, and informing the 30-country bloc of the transit points would increase the risk of such information leaking. However, it also enables NATO to claim credibly that, as a bloc, it is not providing lethal assistance to Ukraine.

This warning, combined with Russia’s recent attack on a military base in western Ukraine, raises questions about Moscow’s willingness to target aid shipments crossing the border from neighboring countries. However, NATO continues to believe that Russia would target shipments still within NATO or European Union member countries.

As a result, the Biden administration is now faced with a delicate balancing act as it considers ways to expedite weapon shipments to Ukraine while also dealing with another concerted push by lawmakers to provide additional defense capabilities, including the S-300.

Meanwhile, lawmakers from both parties are urging the Biden administration to increase its efforts to deliver more weapons to Ukraine.

One lawmaker claimed that the public attention in Congress focused on the Polish MiG-29 jets was misplaced, claiming that Ukraine already has planes that aren’t being used, that it doesn’t have enough pilots to fly more planes, and that the country’s runways have been damaged by Russian attacks.

According to a source familiar with the intelligence, providing MiG fighter jets does not make much operational sense. The aircraft present a large target and are difficult to move, resulting in a slew of logistical, operational, and security issues.