Ukraine's prime minister on Tuesday called for fresh talks with Russia on "neutral territory" as deadly fighting rumbled on in the country's east.
"We invite the Russian Federation to hold serious negotiations on a neutral territory. The US and EU are helping us with this," Arseniy Yatsenyuk was quoted as saying by the Interfax Ukraine news agency.
Meanwhile, Russian President Vladimir Putin said the United States wants to subdue Moscow, but will never succeed.
"They do not want to humiliate us, they want to subdue us, solve their problems at our expense," Putin said at the end of a four-hour meeting with his core support group, the People's Front.
"No one in history ever managed to achieve this with Russia, and no one ever will," he said, triggering a wave of applause.
Putin's tough talk reflected the strain in ties between Moscow and Washington, who are at loggerheads over the crisis in Ukraine, where the West has accused Russia of promoting and arming a separatist rebellion.
Military build-up in Russia-Ukraine border
Russia has launched a "very serious" military build-up inside Ukraine and on their shared border, deploying troops and sophisticated equipment including air defense systems, NATO head Jens Stoltenberg said.
Troops, artillery and "very modern" air defense weapons could be seen "both inside of Ukraine and also on the Russian side of the border," Stoltenberg said as he arrived for a meeting with European Union defense ministers.
"It is a very serious build-up," Stoltenberg said, urging Russia to reverse course and meet its commitments under a September ceasefire and peace plan signed by Kyiv and pro-Kremlin rebels.
"Russia has a choice. Russia can either be part of a peaceful negotiated solution or Russia can continue on a path of isolation," Stoltenberg said.
But for the moment "we see that Russia is still destabilizing Ukraine. We see movement of troops, tanks, artillery, air defence systems in violation of the ceasefire," the former Norwegian premier said.
Air defense systems are a particular concern after the shooting down of Malaysia Airlines flight MH17 over eastern Ukraine in July, which Western governments allege was carried out by pro-Kremlin rebels.
Stoltenberg took office in October with the Ukraine crisis top of his agenda as the US-led military alliance beefed up its readiness in response to Russia's actions which spooked newer NATO members such as Poland and the Baltic states once ruled from Moscow.
He is visiting the Baltic states later this week.
NATO has deployed aircraft and personnel on a rotational basis through its eastern member states in an effort to reassure them and embarked on longer term planning in the face of what it sees as a more assertive Moscow under President Vladimir Putin.
Putin insists Russia has no presence in eastern Ukraine where the rebels appear to be consolidating areas under their control.
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