The House Armed Services Committee has asked the Pentagon to calculate the cost of permanently stationing what are now rotational U.S. military forces in Europe, chairman Rep. Mac Thornberry said Monday.

Thornberry said lawmakers want to see how keeping units on the continent long term compares to the cost of short rotations aimed at reassuring allies over Russia and supporting NATO training operations.

U.S. soldiers arrived in Poland in January and fanned out to sites across Eastern Europe, where concerns run highest over Russia's claims on Crimea and its assistance to rebels fighting the Ukraine government.

"I don't know what the cost data will show," he said. "I'm not convinced that it is tremendously cheaper to rotate a bunch of units through rather than have that permanent presence."

The committee is responsible for the annual National Defense Authorization Act, which sets policy such as the U.S. European presence and is expected to be completed this summer.

But Thornberry said he had no opinion yet on the possibility of a larger permanent presence and was unsure whether any new policy would make it into the bill.

"There is a variety of interests and different options for a more permanent presence for us," he said.

Any increased costs would likely be a major factor. President Trump is set to release his 2018 defense budget Tuesday, and defense hawks such as Thornberry have already criticized it as being far too little to rebuild a military already depleted by its current tasks.

The Obama administration began to beef up the military presence in Europe in 2014 after widespread concern among allies over Russia's annexation of Crimea.

In November, the Army announced it will continually rotate U.S.-based armored brigade combat teams to the continent, where the American bases and military personnel have remained since the end of World War II.

"There is a tremendous interest in Eastern Europe for a more permanent presence," Thornberry said.