KYIV, Feb 1 (Reuters) - Ukraine has started to receive gas from western Europe via Hungary, the operator of its gas transmission system (GTSOU) said on Tuesday, opening a new supply route as the threat of war with Russia undermines Kiev's energy security.

Ukraine needs to import more than 30% of its gas and it piped in 15.9 billion cubic metres (bcm) from abroad in 2020, including 4.2 bcm from Hungary.

But supplies from Hungary stopped after Budapest and Russia's Gazprom signed a new long-term supply deal in September that bypasses Ukraine.

GTSOU said in December that Ukraine would instead be able to import, via Hungary, 8 million cubic metres (mcm) of gas daily from the western European market.

The operator said on Tuesday that that supply line was now active, under an interconnection agreement that would increase Ukraine's guaranteed import capacity by almost 30%.

"The Hungarian route has become the second guaranteed gas supply route to Ukraine after Slovakia," GTSOU head Sergiy Makogon said in a Facebook post.

Ukraine has a deal to get up to 27 mcm daily from Slovakia, but has halted imports via that route on cost grounds.

Makogon said the new route via Hungary also offered Ukraine the opportunity to pipe in liquefied gas from a terminal in Croatia.

Hungarian Prime Minister Viktor Orban said last week he would seek to increase the amount of gas his country receives from Russia at talks with President Vladimir Putin on Tuesday.

Orban also said he would discuss European security with Putin, and that Hungary was interested in a peaceful resolution to the Ukraine standoff.

(Reporting by Pavel Polityuk and Natalia Zinets; editing by John Stonestreet)