NATO Secretary-General Jens Stoltenberg criticized Russia Tuesday amid evidence of a “very serious” military buildup in eastern Ukraine and on the Russian side of the border. Stoltenberg pressed Russian leaders to withdraw their troops and weapons from the region and re-dedicate themselves to the peace process.

“Russia has a choice. Russian can either be part of a peaceful negotiated solution or Russia can continue on a path of isolation,” Stoltenberg said, according to Agence France-Presse. “The international community calls on Russia to be part of the solution.”

NATO claims to have seen an increase in Russian troops, artillery, anti-aircraft weaponry and tanks in eastern Ukraine, Reuters reports. The apparent escalation would violate the terms of a shaky cease-fire agreement in the region. Russia has denied that it sent troops into Ukraine or that it provides armed support to pro-Russian separatists in the war-torn nation.

"But we also see a military buildup on the Russian side of the border...This is a serious military buildup and we call on Russia to pull back its troops," Stoltenberg said.

Stoltenberg’s comments came one week after top NATO commander Gen. Philip Breedlove accused Russia of military aggression in and around Ukraine. “Russian tanks, Russian artillery, Russian air defense systems and Russian combat troops” of unknown intent were seen in the eastern European nation, he said.

Russian Defense Minister Sergei Shoigu denied the charges on Nov. 12, stating that “there was and is no evidence” to support NATO’s claims. Shortly thereafter, Shoigu announced that Russian long-range bomber patrols would extend as far as the Arctic Ocean and the Gulf of Mexico.

The United States and the European Union each levied sanctions against Russian in response to its alleged military involvement in Ukraine. Putin decried the sanctions as illegal ahead of last weekend’s G-20 summit in Australia.

“This contradicts international law, because sanctions can only be imposed within the framework of the United Nations and its Security Council,” Putin said in an interview with Itar-Tass, a Russian state news agency. “This is harmful, and of course is doing us some damage, but it’s harmful for them as well because, in essence, it’s undermining the entire system of economic relations.”