Russia and Ukraine disagree over where their maritime boundary should be drawn through the Kerch Strait, a 41-km (25-mile) channel no wider than 15 km (9 miles) that links the Black Sea to the Sea of Azov.
Lavrov described as "astonishing" statements by Leonid Osavolyuk, director of the first territorial department at Ukraine's Foreign Ministry, who blamed Russia for allegedly attempting to seize a part of Ukrainian territory in the Kerch Strait.
"It is absolutely unprofessional to speak about us moving a certain border in the Kerch Strait, the Sea of Azov or in the Black Sea," Lavrov said late on Sunday, adding that a border must be established before it can be moved.
Negotiations to fix a border in the channel are underway, Lavrov said.
Ukraine's Foreign Ministry banned investment late in October in a project to design and build a bridge across the Kerch Strait until Kiev and Moscow agree on the demarcation of the border.
