The Pentagon is exploring options for deploying U.S. troops to Poland to expand NATO’s presence in Eastern Europe because of the ongoing crisis in Ukraine, as pro-Russian militants defiantly refused Friday to leave government buildings in eastern Ukraine despite a diplomatic accord reached in Geneva.

A senior U.S. official told Fox News on Friday the U.S. is considering sending relatively small units to the country of around 130 soldiers, and the units would be there on a rotational basis.

Poland’s Defense Minister Tomasz Siemoniak told the Washington Post Friday the U.S. was planning on sending ground troops after speaking with Secretary of Defense Chuck Hagel on Thursday.

Siemoniak said military planners are working out the logistics and the two countries also plan to increase their cooperation in air defense, cyber defense, special forces and other areas. Siemonack said he believes the U.S. needs to turn its focus back to Europe, after announcing a “pivot” to Asia.

“The idea until recently was that there were no more threats in Europe and no need for a U.S. presence in Europe anymore,” Siemoniak told the Washington Post. “Events show that what is needed is a re-pivot, and that Europe was safe and secure because America was in Europe.”

Pentagon Press Secretary Rear Adm. John Kirby told Fox News the agency will not announce any specific plans at this time, but is considering military options in Europe.

“As Secretary Hagel made clear, we continue to look for ways to reassure NATO allies of our strong commitment to collective defense under Article Five,” he said. “To that end, we are considering a range of additional measures we could take to bolster air, maritime and ground readiness in Europe.”

Kirby said such measures would either be pursued bilaterally with individual nations or through NATO.

The U.S. also deployed 12 F-16 fighter jets to Poland in recent weeks and delivered 10 F-15s to the Baltic states for air-patrol programs. Vice President Joe Biden said on a trip to Poland last month the U.S. is looking forward to helping Poland as it continues to modernize its military.

“It goes without saying that collective defense is a shared responsibility, and the United States of America strongly supports Poland’s military modernization and we look forward to being a partner in that modernization,” Biden said.

Talks between Ukraine, Russia, the United States and the European Union produced an agreement Thursday in Geneva to take tentative steps toward calming tensions in Ukraine. The country's former leader fled to Russia in February and Russia annexed Crimea in March. The Geneva agreement calls for disarming all paramilitary groups and immediately returning all government buildings seized across the country.

Denis Pushilin of the self-appointed Donetsk People's Republic told reporters on Friday the insurgents in more than 10 cities do not recognize Ukraine's interim government as legitimate and will not leave the buildings until the government resigns. He demanded that Ukrainian leaders abandon their own public buildings.

Ukraine has scheduled a presidential election for May 25, but Pushilin reiterated a call to hold a referendum on self-determination for the Donetsk region by May 11. The same kind of referendum in Crimea led to its annexation by Russia.

In a sign that Ukraine's fledging government is ready to meet some of the protesters' demands, the acting president and prime minister issued a joint statement Friday saying the Ukrainian government is "ready to conduct a comprehensive constitutional reform that will secure powers of the regions," giving them a greater say in local governance.

They also pledged "a special status to the Russian language" and vowed to protect the rights of all citizens whatever language they spoke.

In Washington, President Barack Obama conveyed skepticism about Russian promises to de-escalate the volatile situation in Ukraine, and said the United States and its allies were ready to impose more sanctions if Moscow doesn't make good on its commitments.

Russian President Vladimir Putin's spokesman, Dmitry Peskov, complained on state television about possible further sanctions against Russia.

"You must not act toward Russia as if it were a naughty schoolgirl, to whom you thrust some kind of paper where it's necessary to mark off that she did her homework," Peskov said.

Meanwhile, former Ukrainian prime minister and presidential hopeful Yulia Tymoshenko arrived Friday in Donetsk in a bid to defuse the tensions and hear "the demands of Ukrainians who live in Donetsk."

"I'd like to listen to these demands by myself and find out how serious they are, so that one could find the necessary compromise between the east and the west that will allow us to unite the country," she told The Associated Press.

Russian Foreign Minister Sergey Lavrov has emphasized that the requirement to abandon occupied buildings applied to all parties — an apparent reference to the ultranationalist Right Sector, whose activists are occupying Kiev city hall and a Kiev cultural center.

Fox News' Justin Fishel and The Associated Press contributed to this report.