A top U.S. military official said Russian heavy bombers flew more "out-of-area patrols" last year than in any year "since the Cold War," as he warned Congress about Moscow's increasing "assertiveness" and military advances.

Adm. William Gortney, head of U.S. Northern Command, delivered the warning in written testimony Thursday to the Senate Armed Services Committee. And he said over time, Moscow's aggression could pose "increased risk" to the ability to defend North America against Russian threats.

After a series of reports of Russian flights spotted everywhere from the Pacific to the Gulf of Mexico to off the coast of England, Gortney confirmed that this activity represents a "notable increase in Russian military assertiveness on the world stage, including in the approaches to the homelands."

Aside from the increase in heavy bombers flights, he said Russia is progressing toward deploying "long-range conventionally-armed cruise missiles with ever increasing stand-off launch distances" on those heavy bombers and other systems.

Gortney said this builds the Kremlin's "toolkit" of "deterrent options short of the nuclear threshold."

The warning comes as Russia defies international sanctions and warnings with its intervention in eastern Ukraine, and the Obama administration weighs the possibility of lethal aid for Ukrainian forces.

Asked at Thursday's hearing about Russia's progress in developing more dangerous weapons, Gortney said he's seen a "pretty significant increase" in terms of Russia's long-range aviation.

He said his concerns revolve around where they're flying -- "even through the ... English Channel."

"We have to look at this in a more expansive manner. But if we have the investments that we've asked for, we'll be able to outpace that technology," Gortney said, referencing concerns about tightened Defense funding.

On Friday, Reuters reported that Russia also has rejected U.S. concerns about it using an ex-U.S. base in Vietnam to refuel its bomber flights.

Previously, the U.S. asked Vietnam to stop letting Russia use the base for refueling, according to Reuters. A Russian government statement called the American concerns "puzzling."

But a U.S. military official had told Reuters the Russian planes have conducted "provocative" flights near Pacific U.S. territory including Guam.