The uncertainty over his country’s membership is motivation for Russia to “continue its terror”, the Ukrainian President said as the leaders of the military alliance met in Vilnius, Lithuania on Tuesday.
In a communique, Nato said it recognised the need to move faster but would not be drawn on a timeframe.
Mr Zelensky voiced his anger about the stance of some Nato countries as he was heading for the summit.
He has been pressing Nato for a clear path for Ukraine to join once the war is over.
“We value our allies," he said. “We value our shared security. And we always appreciate an open conversation.
“Ukraine will be represented at the Nato summit in Vilnius. Because it is about respect.
“But Ukraine also deserves respect. Now, on the way to Vilnius, we received signals that certain wording is being discussed without Ukraine.
“And I would like to emphasise that this wording is about the invitation to become Nato member, not about Ukraine’s membership.
“It’s unprecedented and absurd when time frame is not set neither for the invitation nor for Ukraine’s membership. While at the same time vague wording about ‘conditions’ is added even for inviting Ukraine.
“It seems there is no readiness neither to invite Ukraine to Nato nor to make it a member of the Alliance.
“This means that a window of opportunity is being left to bargain Ukraine’s membership in Nato in negotiations with Russia. And for Russia, this means motivation to continue its terror.
“Uncertainty is weakness. And I will openly discuss this at the summit.”
"We reaffirmed Ukraine will become a member of Nato and agreed to remove the requirement for a membership action plan," Nato Secretary-General Jens Stoltenberg told reporters later. “This will change Ukraine's membership path from a two-step path to a one-step path," he said.
The comminique issued by Nato on Tuesday said: “We fully support Ukraine’s right to choose its own security arrangements. Ukraine’s future is in Nato. We reaffirm the commitment we made at the 2008 Summit in Bucharest that Ukraine will become a member of Nato, and today we recognise that Ukraine’s path to full Euro-Atlantic integration has moved beyond the need for the Membership Action Plan.
“Ukraine has become increasingly interoperable and politically integrated with the Alliance, and has made substantial progress on its reform path. In line with the 1997 Charter on a Distinctive Partnership between Nato and Ukraine and the 2009 Complement, Allies will continue to support and review Ukraine’s progress on interoperability as well as additional democratic and security sector reforms that are required.
“Nato Foreign Ministers will regularly assess progress through the adapted Annual National Programme. The Alliance will support Ukraine in making these reforms on its path towards future membership. We will be in a position to extend an invitation to Ukraine to join the Alliance when Allies agree and conditions are met.”
World leaders are discussing more military support for Kyiv at the Nato summit as well as Ukraine joining the military alliance and how it would bolster its defences against future threats.
Opening the summit, Nato secretary general Jens Stoltenberg promised that Ukraine would get a “positive and strong message” on its path to membership of the alliance, as well as more military aid.
But divisions among the alliances 31 member states mean there will not be a straightforward invitation for Ukraine to join.
US national security adviser Jake Sullivan also said the gathering would send a “positive signal” about Kyiv’s bid, though America appears less keen on a fast track approach, with Britain voicing more support for such a path.
He gave the whole-hearted support to Ukraine as he flew out to the summit.
Asked if the UK would continue to support Ukraine even if the war dragged on for another decade, Mr Sunak said: “We are with Ukraine for as long as it takes.”
Mr Zelensky is expected to meet on Wednesday with US President Joe Biden and other Nato leaders.