Russian President Vladimir Putin on Monday inaugurated a massive railway bridge to Crimea, which Russia annexed from Ukraine in 2014.
Putin rode a commuter train for the opening of the 19-km bridge, which is the longest in Europe.
The $3.6-billion project is intended to bolster links between Russia and Crimea by increasing the transport of cargo and encouraging the flow of tourists.
Putin congratulated construction workers on completing the bridge, saying that "it has proven our ability to carry out large-scale infrastructure projects."
Ukraine President Volodymyr Zelensky's representative office for Crimea said in a statement on Facebook that the rail link and Putin's visit to Crimea were "a gross breach by the Russian side of Ukraine's sovereignty and territorial integrity."
It showed "disregard by the Kremlin of the universally recognised principles and norms of international law," the statement said.
He said that 14 million passengers and about 13 million tons of cargo are expected to move across the bridge next year.
While passenger traffic across the bridge was launched on Monday, cargo trains will start rolling across it next July, said Transport Minister Yevgeny Ditrikh.
Russia annexed Crimea in March 2014 on the grounds that the residents of the peninsula voted to join up with Russia.
The move has met with widespread international censure, including US and EU sanctions on Russia.
Ukraine has also blocked shipment of supplies via its territory to Crimea, and a ferry crossing from Russian Black Sea ports is often interrupted by gales.
Car traffic across a parallel bridge for cars across the Kerch Strait between the Black Sea and the Sea of Azov opened in May 2018.