There are informal agreements between NATO countries to refrain from supplying certain weapons systems to Ukraine, German newspaper Die Zeit reported on May 25, citing their own sources.
According to the publication, alliance members in Brussels have said that threats of military escalation between NATO and Russia need to be minimized as much as possible, and that Russia may consider the delivery of NATO tanks and warplanes as escalation, with some members worried that Russia may take action in response.
No such systems have been delivered to Ukraine as of yet. Since the beginning of the war, Ukraine has been asked for fighter jets in order to protect its skies from Russian airstrikes.
Meanwhile, Russia has already threatened supply lines from Poland to Ukraine with military action, but has so far taken none.