"We need [an] invitation" to join the alliance, Mariya Ionova of Ukraine's European Solidarity party told CBC Radio's The House on Saturday. She said the fact that Ukraine has been waiting so long for that invitation strains the credibility of NATO's so-called open door policy.

"If the door is open, why have we been standing on the threshold for 15 years?" she said.

Some of NATO's 31 member states are reluctant to let Ukraine in because they're afraid of further aggravating tensions with Russia.

The alliance considers the idea of admitting Ukraine while it's at war a non-starter. At a minimum, however, President Volodymyr Zelenskyy's government expects to be presented with a pathway to membership.

Stoltenberg hinted Friday that the allies are engaged in backroom talks on the topic.

"We are helping Ukraine to come even closer to NATO and NATO membership," the secretary general said.

"I am confident that we'll find the united way also to address the specific issue on membership, but I will not go into the details of exact language now because that is something we will announce when everything is in place by the summit."

And this is where Ukrainian MPs are counting on Trudeau and Canada to step up and play a pivotal role.

"I think that Mr. Trudeau can talk to [those] countries, to the leaders of [those] countries which are not sure in their decision, which are not sure to support or not to support," said Lesia Zabruanna, a member of Ukraine's governing Servant of the People party.

Hungarian Prime Minister Viktor Orbán, centre, addresses supporters after the announcement of the partial results of the parliamentary election, in Budapest on April 3, 2022. (Leonhard Foeger/Reuters)

Hungary is one of the NATO countries with doubts about admitting Ukraine. Hungarian Prime Minister Viktor Orbán last week rejected the European Commission's plan to grant more money to Ukraine. He went further in an interview with a German publication.

"Ukraine is no longer a sovereign state. It has neither money nor weapons. It can keep on fighting only thanks to the assistance being provided," Orban told the German daily Bild.

"It's up to the United States to decide when peace will materialize."

Orban's remarks were widely reported in Russia and drew sharp condemnation from Kyiv.

Canada favours a shortcut for Ukraine

Last spring, Orban cast doubt on whether his country would approve long-term NATO membership for Ukraine.

Stoltenberg is on record saying that some NATO nations would favour Ukraine's direct admission without the preliminary step of creating a membership action plan.

Senior Canadian government officials, speaking on background Friday, said Canada is among those in favour of dropping the preliminary step.

Retired U.S. lieutenant-general Ben Hodges said the United States has not offered a firm endorsement either way.

"There's a difference between invitation and joining," said Hodges, who commanded the U.S. Army in Europe.

An invitation without clear timelines "provides an opportunity for the Russians" both from a propaganda and political perspective, he said, "but a firm invitation could be that sort of a clear signal" to Moscow that there's no turning back on Ukraine's membership.

Stoltenberg made the announcement after talks with Erdogan and Swedish Prime Minister Ulf Kristersson on the eve of a North Atlantic Treaty Organization summit in Lithuania. Sweden's NATO accession has been held up by objections from Turkey since last year.

"This is an historic day because we have a clear commitment by Turkey to submit the ratification documents to the Grand National Assembly, and to work also with the assembly to ensure ratification," Stoltenberg told reporters.

Earlier Monday, with NATO hoping to put on a public display of unity in its support for Ukraine more than 500 days after it was invaded by Russia, Erdogan said he would block Sweden's path unless European members of the military organization "pave the way" for Turkey to join the world's biggest trading bloc.

His surprise announcement added new uncertainty to Sweden's bid to become the alliance's 32nd member. Turkey was already blocking its entry because Erdogan believes that Sweden has been too soft on Kurdish militants and other groups that he considers to be security threats.

On arriving in the Lithuanian capital, Vilnius, where Canadian Prime Minister Justin Trudeau and his counterparts will hold two days of talks starting on Tuesday, Erdogan first met with Kristersson, before breaking off for a separate meeting with European Council President Charles Michel.

Michel said in a Twitter post that he and Erdogan had "explored opportunities ahead to bring co-operation back to the forefront and re-energize our relations." Michel said he has tasked the European Commission to draw up a "report with a view to proceed in strategic and forward-looking manner."

Turkey is a candidate to join the European Union, but its membership talks have been at a standstill since 2018 due to democratic backsliding during Erdogan's presidency, concerns about the rule of law and rights abuses, and disputes with EU-member Cyprus.

Erdogan's new demand was the first time he had linked his country's ambition to join the EU with Sweden's efforts to become a NATO member.

Flags of NATO member countries are seen outside a venue to be used for a NATO summit in Vilnius, Lithuania, on Sunday. (Mindaugas Kulbis/The Associated Press)

"Turkey has been waiting at the door of the European Union for over 50 years now, and almost all of the NATO member countries are now members of the European Union," Erdogan told reporters in Istanbul.

"I am making this call to these countries that have kept Turkey waiting at the gates of the European Union for more than 50 years.

"Come and open the way for Turkey's membership in the European Union. When you pave the way for Turkey, we'll pave the way for Sweden as we did for Finland," he said.

Earlier, Erdogan's office said he told U.S. President Joe Biden during a telephone call on Sunday that Turkey wanted a "clear and strong" message of support for Turkey's EU ambitions from the NATO leaders. The White House readout of the Biden-Erdogan call did not mention the issue of Turkish EU membership.

'This is a matter of timing'

Erdogan's government has postponed ratifying Sweden's accession, saying the administration in Stockholm needs to do more to crack down on Kurdish militants and other groups. A series of anti-Turkey and anti-Islam protests in Sweden's capital raised doubts that an agreement to satisfy Turkey's demands could be reached before the NATO summit.

Turkey's delaying tactics have irritated other NATO allies, including the United States.

Biden's national security adviser, Jake Sullivan, confirmed on Sunday that Biden and Erdogan had spoken about Sweden's NATO membership among other issues and had agreed to meet in Vilnius for further talks.

     A Canadian soldier wears a NATO patch during an address by Prime Minister Justin Trudeau at a military base in Adazi, Latvia, on Monday. (Adrian Wyld/The Canadian Press)

Sullivan said the White House is confident Sweden will join the alliance.

"We don't regard this as something that is fundamentally in doubt. This is a matter of timing. The sooner the better," he said.

Previously non-aligned Sweden and Finland applied for NATO membership last year following Russia's invasion of Ukraine. Finland joined in April following Turkish ratification.

Another key issue in Vilnius will be how to bring Ukraine closer to NATO without actually joining, as well as security guarantees Kyiv might need to ensure that Russia doesn't invade again after the war ends. Ukrainian President Volodymyr Zelenskky will join the summit in person on Wednesday.

Stoltenberg said the most important thing was to continue to support Ukraine's efforts to resist the Russian invasion.

"Unless Ukraine prevails, there is no membership issue to discuss at all," he said.