At an emergency United Nations Security Council meeting on Tuesday convened at Russia’s request, Moscow’s envoy in New York called the crisis in eastern Ukraine a “humanitarian catastrophe.” Ukrainian forces are closing in on the remaining pro-Russian strongholds in Ukraine’s east, and the situation appears increasingly tenuous for the pro-Moscow separatists.

With tens of thousands of Russian troops stationed along the country’s western border, the Kremlin is flirting with the notion of sending a humanitarian mission to intervene in Ukraine. This has sparked fears in the West that Russia is planning to use the humanitarian condition in Ukraine as a pretext for a military invasion of portions of that country.

“We would like to send a convoy with Russian humanitarian assistance under the egis and with the accompaniment of the ICRC to Donetsk and Luhansk, as well as to other inhabited areas of Ukraine where IDPs are concentrated from the east,” said Russian Ambassador Vitaly Churkin.

“Russia has amassed around 20,000 combat-ready troops on Ukraine’s eastern border and could use the pretext of a humanitarian or peace-keeping mission to invade, NATO said on Wednesday,” Reuters reported.

“We’re not going to guess what’s on Russia’s mind, but we can see what Russia is doing on the ground – and that is of great concern. Russia has amassed around 20,000 combat-ready troops on Ukraine’s eastern border,” NATO spokeswoman Oana Lungescu said in an emailed statement.

NATO was concerned that Moscow could use “the pretext of a humanitarian or peace-keeping mission as an excuse to send troops into Eastern Ukraine”, she said.

Polish Prime Minister Donald Tusk also warned that Russia could be preparing to intervene directly in the fighting in Ukraine. He noted on Wednesday that the risk of an invasion of eastern Ukraine “is for sure higher than it was several days ago.”

Russia is currently conducting extensive military exercises on the Ukrainian border. American officials are “deeply concerned” about aviation operations Russia is conducting along its Western border which are expected to continue until the end of the week.

“A NATO military officer, speaking on condition of anonymity, said that early this month Russia had significantly increased the number of troops near the Russian border,” another Reuters report published Wednesday revealed.

The roughly 20,000-strong troop presence included tanks, infantry, artillery, air defense systems, as well as logistics troops, special forces, and various aircraft, the officer said.

Previously, Russia has used military exercises as cover for intervention, military analysts say.

In spite of the sanctions imposed on Russia by the United States and European Union member states, President Vladimir Putin and his allies have shown no signs of backing down from their gambit in Ukraine.

In response to the sanctions, Putin and top officials announced plans to hold a legislative session in the occupied Ukrainian peninsula of Crimea next week, are reportedly considering banning flights over Russian territory by European airlines, and have banned the import of a variety of foods from nations participating in sanctions.