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Defense Secretary Chuck Hagel has been trying to get Russian Defense Minister Sergei Shoigu on the phone the past 24 hours to discuss de-escalating the tensions along the Russian border but has been unable to reach him, Pentagon officials said today.

And the U.S. military continues to see Russian troops leaving their bases to conduct newly announced exercises close to the border with Ukraine, officials said. And tensions continue to rise as the Pentagon confirmed Friday that over the past 24 hours Russian aircraft have entered Ukrainian airspace.

"We have made it clear to the Russians that the Secretary is willing to speak to them," Pentagon spokesman Col. Steve Warren told reporters today.

Hagel's office reached out to the Russian Defense Ministry in the past 24 hours to communicate that Hagel would like to speak to Shoigu, Warren said. "Now we're waiting to hear from the Russians," he said.

However, the Russian Defense Ministry said they had not received a notice that Hagel wished to speak with Shoigu.

"The Russian Defense Ministry was surprised by reports in the press citing Hagel's apparent attempts to organize a telephone conversation with Sergei Shoigu," ministry spokesman Maj. Igor Konashenkov told Voice of Russia.

Warren said that Hagel wants to speak with Shoigu "to continue calling on the Russians to de-escalate the situation in Ukraine."

"Their continued destabilizing activities along the Ukrainian border are unhelpful and they need to withdraw their troops from the Ukrainian border and place them back into their garrisons and go about working towards a peaceful resolution of this crisis," Warren added.

While Hagel's not been able to get in touch with Shoigu, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, spoke Thursday with his counterpart Gen. Valery Gerasimov. No details of the call were provided by a spokesman.

Over the past month, Russia has dispatched as many as 40,000 troops to the border region with Ukraine for what it called exercises, but which were seen as a provocation by the U.S. and NATO. Satellite imagery released by NATO two weeks ago showed large numbers of Russian military equipment, vehicles and aircraft that had gathered at six bases along the border.

Earlier in the week, Russia announced the forces would engage in new exercises as a response to Ukrainian military operations against pro-Russian separatists in eastern Ukraine and to new NATO exercises in eastern Europe.

At a Mexico City news conference on Thursday, Hagel labeled the new Russian exercises "dangerously destabilizing" and "provocative." He added that Russian activities along the border with Ukraine are making it harder to find a diplomatic resolution.

The U.S. is "continuing to see troop movements across the region," Warren said Thursday, indicating the Russian military is "beginning to conduct maneuvers" consistent with the newly announced exercises.

Thousands of Russian forces have been seen leaving their bases to participate in the maneuvers, he said, describing a "a broad array of forces" participating in the exercises, including mechanized infantry, armor and airborne forces, as well as Russian fixed-wing aircraft and helicopters.

And some of those aircraft have entered Ukrainian airspace, Warren confirmed that "on several occasions in the last 24 hours, Russian aircraft have entered Ukrainian airspace." Warren said, " We call upon the Russians to take immediate steps to de-escalate the situation. "

A defense official told ABC News that the new Russian exercises are taking place in areas closer to the border than the Russian military bases, one of which is located just 25 miles from the border.

Secretary of State John Kerry delivered a strong warning to Russia on Thursday that the United States was ready to impose a new round of sanctions against Russia unless it de-escalates the tension in eastern Ukraine.

"The window to change course is closing," Kerry said. "If Russia does not, the world will make sure that the costs for Russia will only grow."

The Pentagon announced earlier this week that 600 U.S. Army paratroopers would be sent to Poland, Lithuania, Estonia and Latvia to participate in new exercises intended to show the U.S. commitment to NATO's defense. Of those soldiers, 150 arrived in Latvia today. The first company of 150 had arrived in Poland on Wednesday.

The U.S. commitment to regional security has included dispatching Navy ships to the Black Sea to engage in exercises with regional partners. The frigate USS Taylor is now in the Black Sea for unspecified exercises replacing the destroyer USS Donald Cook, which had carried out the same role for the past two weeks.