The Three Seas Summit and Business Forum was expected to focus on issues including Russia's war against Ukraine and Ukraine's ties with the regional group.
Leaders from across Central and Eastern Europe as well as further afield were due to attend.
The Polish-led Three Seas Initiative aims to boost infrastructure, energy and business ties among 12 countries between the Baltic, Black and Adriatic Seas.
The initiative brings together Poland, Austria, Bulgaria, Croatia, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Romania, Slovakia and Slovenia.
The initiative’s key partners are Germany, the European Union and the United States.
Speaking earlier this month, during a conference of local government officials from the Three Seas region in the eastern Polish city of Lublin, Ukraine’s departing ambassador to Poland, Andrii Deshchytsia, said his country was interested in becoming a member of the Three Seas Initiative.
He argued at the time that Ukraine’s entry would strengthen the regional club and widen its possibilities when it comes to logistics, infrastructure, security, financial turnover and transport.
One country that has fully supported the plan is Lithuania.
"The Three Seas Initiative works by consensus so we need to have all 12 countries on board," Gediminas Varvuolis, Ambassador at Large at the Lithuanian Ministry of Foreign Affairs, who is responsible for the Three Seas Initiative at its Europe department, told Radio Poland's Danuta Isler.
He added: "This discussion will take place in Riga. I think one way or another Ukraine will get closer because there is a kind of cultural and historical affinity between the Three Seas Initiative countries and Ukraine. There is also a lot of understanding and sympathy towards Ukraine, which is now fighting against the Russian invader."
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