The Kremlin has “little ground for optimism” about ending the conflict with Ukraine, spokesman Dmitry Peskov said Thursday.

The comments come as the U.S. has threatened economic sanctions against Russia for its military buildup along the border and after rejecting Russia’s demands that Ukraine not be admitted to NATO.

The Defense Department said on Thursday that Russia has increased forces near the Ukrainian border “in the last 24 hours.”

Tensions have escalated in recent weeks as Russia has built up more than 100,000 troops on the border with Ukraine. There are about 8,500 U.S. troops on high alert for possible deployment to Eastern Europe should Russia decide to invade.

“The United States has taken steps to heighten the readiness of its forces at home and abroad, so they are prepared to respond to a range of contingencies, including support to the NATO response force if it is activated," Pentagon spokesman John Kirby told reporters. He noted the NATO Response Force "comprises around 40,000 multinational troops."

Some analysts have feared that the build-up of NATO troops near Russia’s border could create a pretext for a Russian invasion.

Russia has repeated that it has no plans to invade Ukraine.

“We have already repeatedly stated that our country does not intend to attack anyone. We consider even the thought of a war between our people to be unacceptable," Foreign Ministry spokesman Alexei Zaitsev said this week.

Dimitry Medvedev, deputy chairman of the Russian council, warned that tensions could become “seriously complicated” if Ukraine was not barred from entering NATO. Medvedev said no one is looking for war but Russia has “exhausted the limits of retreat,” adding that NATO is “encroaching on our state borders.” 

The Kremlin has accused NATO of aggression over its military buildup.

Secretary of State Anthony Blinken said on Wednesday that the U.S. response to Russia’s demands was a “serious diplomatic path forward” to deescalating tensions. President Biden was “intimately involved” in their response, which included European allies.  

"NATO's door is open, remains open and that is our commitment,” Blinken said at a press conference.

“The ball is in their court, it remains up to Russia how to decide to respond."