The United States on Thursday sent six additional F-15 fighter jets to step up NATO's air patrols over the Baltic states, mission host Lithuania said as West-Russia tensions simmered over Ukraine.

"I have had confirmation that the air police missions will be reinforced by six additional F-15 fighters," Defence Minister Juozas Olekas told AFP.

The move is a response to "Russian aggression in Ukraine and additional military activity in the Kaliningrad region," Russia's exclave bordering Lithuania and Poland, he said.

"We have witnessed increased military activity in Kaliningrad. Today it is less than three or four days before."

The jets took off from the US-run Lakenheath air base in eastern England and landed on Thursday afternoon at Lithuania's Zokniai air base, once the home of Red Army troops near the northern town of Siauliai, the ministry said in a statement.

Lithuanian President Dalia Grybauskaite told reporters in Brussels that the jets are a sign that "NATO is responding promptly and fast".

"Europe still is not able to understand what is happening," she said.

"Russia today is dangerous. Russia today is unpredictable."

Since January, four US F-15 fighter jets have been assigned for air patrols over Estonia, Latvia and Lithuania -- three ex-Soviet Baltic states which are members of NATO but which lack sufficient aircraft to patrol their skies.

The countries broke away from the crumbling Soviet Union in 1991 after five decades of Communist rule and joined NATO in 2004.

They have repeatedly voiced their concern at the Russian military build-up near their border -- and the escalating crisis in Crimea has added to that unease.

Grybauskaite on Wednesday urged NATO to increase its "visibility in the Baltic states".

Defence ministry spokeswoman Viktorija Cieminyte told AFP that NATO had scrambled jets more than 40 times last year in response to the increased number of flights of Russian aircraft near the Baltic states' borders.

NATO also sent more fighters to identify Russian aircraft in January and February than in 2012, she said, declining to provide specific numbers.