(Updates with briefing details)

By Phil Stewart and Idrees Ali

WASHINGTON, Feb 23 (Reuters) - Russian President Vladimir Putin has put 80% of the troops he has assembled in a position to launch a full-scale invasion on Ukraine, a senior U.S. defense official said on Wednesday.

"He is as ready as he can be," the official told reporters, speaking on condition of anonymity.

The remarks added to a sense of imminence of a possible Russian military onslaught, challenging any notion that Washington expected Putin to stop after recognizing the independence of two Moscow-backed rebel regions this week.

"They have advanced their readiness to a point where they are literally ready to go - now - if they get the order," the official said, without providing evidence to support the assertion.

The United States has assessed that more than 150,000 Russian forces are assembled around Ukraine. The senior U.S. official said the Russian troops had taken ready positions, with some as little as 5 kilometers (three miles) from Ukraine's border.

The Russian forces are largely land-based troops, including more than 120 battalion tactical groups. But Putin has also deployed more than two dozen warships in the Black Sea, including landing ships with Marines aboard, and counts significant artillery and missile forces, the official said.

To date, the Russian military has assembled nearly 100% of the forces that the United States anticipated it would mobilize for a large-scale attack, the official said.

The United States has been troubled by indications that Russia plans to mobilize its reserve forces.

"We do have indications that they plan to use reserves and their equivalent of the National Guard and that is concerning because that would connote to us ... long-term goals," the official said. (Reporting by Phil Stewart and Idrees Ali; Editing by Howard Goller)