COPENHAGEN, Denmark (AP) — Finland’s national carrier Finnair said Monday it is temporarily suspending flights to Estonia’s second largest city for a month after two of its planes were prevented from landing in Tartu because of GPS disruptions.

The cause of the GPS interference that forced the two flights to return to Helsinki last Thursday and Friday was not immediately known, but Estonian officials blame GPS jamming in the region on Russia.

Plane approaches to Tartu Airport currently rely on GPS signals, said Finnair, which is the only airline to fly into that city. But there there are other navigational tools that can be used, and the airline said it would suspend daily flights there from April 29 to May 31 so that an alternate solution can be installed at the airport.

“Most airports use alternative approach methods, but some airports, such as Tartu, only use methods that require a GPS signal to support them,” said Jari Paajanen, Finnair’s director of operations.