Washington (AFP) - The United States on Thursday said Russia had boosted its military flights near US shores and brusquely warned Moscow to abide by international law.

Washington's concerns came only days after a European think tank warned that since Russia annexed Ukraine's southern Crimea peninsula earlier this year, "the intensity and gravity of incidents involving Russian and Western militaries and security agencies has visibly increased."

The European Leadership Network detailed at least nine incidents over the past few months of either near misses between Russian and North American aircraft or cases where Russian planes have been observed close to US territory.

In early September, Russian strategic bombers near Canada practiced cruise missile strikes on the United States, although they stayed outside of Canada's air defense zone.

"While we recognize the need for routine military training activity, we have noticed an increase in the number of these flights near North America in recent months," State Department spokeswoman Jen Psaki told reporters.

"Any such activity must be consistent with international law and conducted with due regard for the rights of other nations and the safety of other aircraft and of vessels," she warned.

The European Leadership Network said in its report released on Monday that in the past eight months there had been 11 serious incidents worldwide "of a more aggressive or unusually provocative nature, bringing a higher level risk of escalation."

"Even though direct military confrontation has been avoided so far, the mix of more aggressive Russian posturing and the readiness of Western forces to show resolve increases the risk of unintended escalation and the danger of losing control over events," it said.

Psaki dismissed reported comments from the Russian defense minister that Moscow needed to maintain a military presence in the Western Atlantic and Eastern Pacific.

"We don't see the security environment as warranting such activity," Psaki said.