VILNIUS- Six American F-15 jets landed in an Estonian airbase on Wednesday for deployment for a week and a half, as the NATO transatlantic military alliance reinforced its eastern flank due to frictions with Russia over Ukraine.

NATO said on Monday it was putting forces on standby and reinforcing eastern Europe with more ships and fighter jets, in what Russia denounced as Western "hysteria" in response to its build-up of troops on the Ukraine border.

The Monday announcement did not mention the U.S. jets being deployed in Estonia.

"The six fighter jets will be deployed until the end of next week", said a spokesperson at the Amari airbase in Estonia.

The F-15s will enhance NATO's Baltic Air Policing mission, said NATO's Allied Air Command.

Estonia, Latvia and Lithuania - Baltic countries once ruled from Moscow but now part of both NATO and the European Union - do not operate fighter jets and rely on NATO to police their airspace.

NATO has previously announced that Denmark will deploy four F-16 fighter jets to a nearby Lithuanian airbase in Siauliai on Thursday.

"The aircraft plan to work with other Allied nations throughout the Baltics practicing air-to-air and air-to-ground training manoeuvres in addition, to the Air Policing missions", the Allied Air Command said in a statement.

Tensions are running high after Russia massed an estimated 100,000 troops within reach of its neighbour's border, surrounding Ukraine from the north, east and south.

(Reporting by Andrius Sytas in Vilnius; Editing by Andrew Cawthorne and Bernadette Baum) ((; +370 682 74006;))