KAISERSLAUTERN, Germany – Polish government officials said Monday that the U.S. military was sending 12 F-16 fighter jets and about 300 service members to their country in response to the situation in Ukraine.

Some U.S. aircraft and service members had already arrived in Poland on Monday, with the remainder expected later in the week, a spokeswoman for Poland’s Defense Ministry said.

Where the warplanes and personnel were coming from is not known, however, as U.S. military officials provided few details on the mission.

It’s the second time in less than a week that the Pentagon has ordered combat planes and personnel to countries in Eastern Europe amid mounting tensions over Russia’s incursion into Ukraine’s Crimea Peninsula. Last Thursday, the U.S. Air Force sent six F-15C Eagles and more than 60 U.S. airmen from RAF Lakenheath, England, to Lithuania to bolster NATO’s air policing mission over the Baltics.

The Baltic nations and Poland requested the deployments, officials said.

Poland requested to speed up a previous planned rotation of U.S. military aircraft in connection with the crisis on its border, said Arthur Golawski, a spokesman for the Polish armed forces.

Normally, U.S. Air Force rotations in Poland are about two weeks. It’s not clear how long this rotation will last. “If it needs to be prolonged, it can be prolonged,” Golawski said. “It’s up to the Americans and how much they will spend on this rotation.”

The U.S. service members and aircraft that have already arrived in Poland are on the ground at Lask air base, said the Polish Defense Ministry spokeswoman. The base, located about 100 miles southwest of Warsaw, has been home to a U.S. Air Force aviation detachment since November 2012. She said the planes and personnel could be moved to several other military bases in Poland that are prepared to receive them.

The spokeswoman, who declined to give her name because she was not authorized to speak on the matter, said an exercise with U.S. forces “was already planned but not at this scope.” She said Americans “responded very quickly” to Poland’s request to expand the military drills.

Details of the type of joint training to be conducted by the U.S. and Poland would be announced later in the week, she said.

A spokesman with U.S. European Command on Monday confirmed the U.S. military was working with Poland “on increasing activities associated with the aviation detachment” but said details were still being negotiated.

“It is too early to talk specific aircraft or quantity,” Lt. Col. David Westover said in a statement. “We’re in consultations to determine what they’d like, what the airfield can handle, and what we can provide.”

A rotation of three C-130s and about 100 personnel from Ramstein Air Base were scheduled to arrive in Poland in early April for joint training, U.S. Air Forces in Europe officials said last week.

Now, F-16s will be arriving earlier as part of an effort to boost activities in response to the crisis in Ukraine, Golawski said.

Last year, the Air Force did four rotations in Poland, two of which involved F-16s and two of which involved C-130s.

Russian forces have seized control of key areas in Crimea, and a pro-Russian government there plans to hold a referendum Sunday on whether the region should secede and join the Russian Federation. The referendum has been widely denounced by European leaders and the United States.

Tens of thousands of people on both sides of the referendum protested over the weekend, with some rallies turning violent, according to news reports.