PRAGUE, July 6 (Reuters) - Ukrainian President Volodymyr Zelenskyy called on NATO leaders to take concrete steps towards Ukrainian membership at a summit next week, and he received support during a visit to Prague from the Czech president, who backed Kyiv's bid to join the alliance.

He said his country, engaged in the 17th month of a war against Russian invaders, needed much more than the general statement of more than 10 years' standing that the door to NATO was merely "open".

Ukraine is seeking a clear indication from NATO at a Jul 11-12 summit in Vilnius that it can join the military alliance when the war ends.

Ukraine wants to join as quickly as possible, but NATO members have been divided over how fast that step should be taken. Some member countries are wary of moves they fear could take the alliance closer to an active war with Russia.

"We are talking about a clear signal, some concrete things in the direction of an invitation," Zelenskyy told an news conference alongside his Czech counterpart, Petr Pavel. "We need this motivation. We need honesty in our relations."

Zelenskyy said the formulation adopted by a 2008 NATO summit was not sufficient - that the military alliance's door "will remain open" and that Ukraine would eventually become a member.

"... We need some kind of signal, a clear one," he said in response to journalists' questions. "That Ukraine will be in the alliance. Not that the door is open - this is not enough."

The Czech Republic, a NATO member, has been a staunch supporter of Ukraine since the invasion in February 2022, supplying military aid and other help.

"It should be made absolutely clear that it is in the interest of the Czech Republic that Ukraine, as soon as the war is over, should start negotiations on joining NATO, because it is also in the interest of our security, regional stability, and economic prosperity," Pavel said.

Pavel, who was Czech army chief and principal military adviser to the NATO secretary-general in 2015-2018, said in a Reuters interview in May that Ukraine needs support for NATO and European Union entry but joining either would be a lengthy process.

Zelenskyy landed earlier in the evening in a Czech government plane, escorted by two Air Force jets, television footage showed. His arrival followed a visit to Bulgaria for talks on security and the NATO summit.

Zelenskyy's diplomatic adviser Ihor Zhovkva said Kyiv had secured the backing of Bulgaria for its membership of the military alliance "as soon as conditions allow".

During the visit, Zelenskyy urged Sofia to continue military support and moved to dispel the notion that supplies of Western weapons to his country had expanded the scale of the war.