Russia has amassed “significant military capacity” on Ukraine’s borders, a top U.S. diplomat confirmed Monday amid fears Russia has its eyes on another piece of the country.

“That is all true,” U.S. Special Representative to Ukraine Kurt Volker told reporters Monday.

Volker is leading State Department efforts to resolve the four-year conflict stemming from Russia’s annexation of Crimea and invasion of eastern Ukraine. Reports about a Russian military buildup have led to worries about a new “land grab” by Kremlin forces, particularly after the seizure of three Ukrainian vessels. The ships tried to sail to a port city that can only be accessed through the Kerch Strait waterway, which is controlled by Russia.

Volker said the buildup has been happening for years.

“It’s not something that has happened in the last week or two,” he told reporters. “This is something that has happened over a period of years. Russia invaded Ukraine, it took Crimea, it is occupying the Donbas, and it has, within Russia, built up significant military capacity and also in Crimea built up significant military capacity over time.”

Still, that military presence could give Russia a tactical advantage in a renewal of heavy fighting with Ukrainian forces. U.S. and regional officials have been on the watch for such an outburst for a variety of reasons, including the Kerch Strait incident and the effort to transfer control of Russian Orthodox churches in Ukraine into the hands of a new national church body.

"I have been and still am concerned that [Russian President Vladimir] Putin will attempt to move even further into Ukraine," Rep. Brendan Boyle, a member of the House Foreign Affairs Committee, told the Washington Examiner last month.

It’s a bipartisan worry.

“It’s pretty obvious what Putin’s doing,” Wisconsin Sen. Ron Johnson, who chairs the Foreign Relations subcommittee for Europe, told the Washington Examiner. “Now the next step for Putin is, let's start choking off maritime activity to those Ukrainian ports to dramatically harm and effect the economy in those two port cities, which could potentially set up the next step of the land bridge from Russia into Crimea.”

Still, Ukraine is not an “easy target” for Russia, because the Kiev government also has upgraded their military capacity in the region, according to a leading defense official from a NATO member-state in the region.

“The Ukrainian military today is very different from the military that they had in 2014,” Kristjan Prikk, the top civilian in Estonia’s ministry of defense, told the Washington Examiner last month. “The Ukrainians have built, bought, [had] donated quite a lot of equipment. They've been putting heavy emphasis on mobility — anti-armor capabilities, communications ... It's definitely a credible fighting force.”