U.S. and European military planners have begun exploring post-conflict security guarantees for Ukraine, U.S. officials and sources told Reuters on Tuesday, following President Donald Trump's pledge to help protect the country under any deal to end Russia's war.

The US president says Russia's Vladimir Putin would face a "rough situation" if he doesn't co-operate in the peace process.

Russia will only agree to security guarantees for Ukraine if it has the ability to veto them, its foreign minister has said.