What is the S-300 missile that is reported to have hit Poland?

Poland has said the missile that hit a grain facility on Tuesday, killing two people near the border with Ukraine, was probably an "old" S-300 rocket, a Soviet-era missile system being used by both Russia and Ukraine.

NATO chief Jens Stoltenberg said it was likely being used as a Ukrainian air defence missile, but that Russia carried ultimate responsibility for the incident for having invaded Ukraine, triggering a now almost nine-month-old war. Russia at the time was firing scores of missiles at cities across Ukraine.

WHAT IS THE S-300?

The S-300...