Swiss voters are casting ballots in a referendum to decide whether to enact new restrictions on guns and line up with Switzerland's partners in the European visa-free travel zone who have already tightened gun rules following extremist attacks in Europe.

The proposal could require regular training on the use of firearms, special waivers for possession of some semi-automatic weapons and serial-numbering of major parts of some guns to help track them.

Supporters, including the Swiss parliament and executive branch, say similar measures adopted by the European Union after deadly extremist attacks in France are needed to ensure strong police cooperation and economic ties with Switzerland's partners in Europe's Schengen zone of visa-free travel.

Switzerland is in the Schengen zone but is not one of the EU's 28 nations.