President Volodymyr Zelenskyy and some of Ukraine’s main European Union backers are weighing options to fill possible gaps after the Trump administration paused weapons shipments to his country.

Analysts say Ukraine’s European allies can fill some of the gaps and provide artillery systems. But they don’t possess alternatives to the U.S.-made HIMARS missiles and air defense systems, especially Patriots, which are crucial to help defend Ukrainian cities.

Calls are mounting for the Europeans to use Russian assets that they froze after the full-scale invasion in 2022 to help Ukraine. At the end of March, around $320 billion worth — the bulk of the assets — was being held by Belgian clearing house Euroclear.