US president Joe Biden’s promise to supply Ukraine with Soviet-era long-range missile systems has become bogged down in difficulties with sourcing and moving the equipment, according to US and western officials.

Even as the US and its allies prepare to announce fresh sanctions on Moscow and additional lethal and humanitarian aid for Ukraine this week, the provision of the anti-aircraft ordnance seen by Kyiv as crucial to its ability to halt Russia’s advance is set to take more time.

The effort is running into similar issues as the now-abandoned plan to transfer Polish fighter jets to Ukraine, a move that descended into a diplomatic fiasco. The countries that have the equipment — Slovakia, Greece and Bulgaria — have either balked at the plan or want new systems as a substitute. They also fear that Moscow will interpret any transfer as an aggressive act.