Ukrainian Ambassador to the United Nations Yuriy Sergeyev said Thursday that Russia has engaged in a full-blown invasion of Ukraine, despite Russia’s denial that it has employed its military in a bid to take over more Ukrainian territory.

“Contrary to its statements, Russia has a launched direct military invasion of the mainland of Ukraine with its regular armed forces,” Sergeyev told reporters. “Military convoys cross Ukraine’s state border and move inside the territory of Ukraine.”

“Currently, those mentioned combat units which crossed the Ukrainian border have occupied several villages and towns… and constructed fortified strongholds in Ukrainian territory,” he added.

Russia took control of the Crimean Peninsula earlier this year, and Sergeyev said he agreed with the speculation that Russia may be looking to control Ukrainian territory along the Sea of Azov in order to create a land bridge to Crimea.

“There is [an] allegation that Moscow is trying to build a corridor to Crimea,” he said. “They need this corridor because Crimea is isolated from the world.” He added that Crimea no longer has access to electricity and water that it once received from Ukraine, which could be another reason why Russia is looking to carve out more territory.

He said that on Thursday morning, Russian tanks rolled into Mariupol, a city on the Sea of Azov near the border of Russia and Ukraine. “The latest Russian actions demonstrate the preparation for a broader, large-scale occupation under the Russian control,” he said.

Sergeyev also said that Russian troops in Luhansk and Donetsk — two Ukrainian cities also close to Russia — have begun issuing people Russian passports. “This is being done to create grounds for Russian authorities to use its forces in Ukraine under the guise of protecting its so-called nationals,” he said.

“In view of these difficult circumstances, we have to request for the international economic and financial systems to provide aid to Ukraine, to help stand against foreign aggression,” Sergeyev said. “We call on the international community for… pressing the Russian Federation in order to make it stop its aggression and its absolutely unacceptable politics in the 21st century.”

Despite his pleas, the U.S. State Department has so far refused to say that Russia has invaded Ukraine.

“I think in our view, it doesn’t matter what we call it,” State Department Spokeswoman Jen Psaki said Tuesday. “We’re calling it an illegal incursion, we’re saying they’re violating the sovereignty of Ukraine.”