The U.S. office market improved in 2011 to its highest leasing level in four years, according to real estate brokerage Cushman & Wakefield.

There was a total of 76.5 million square feet in new leases signed in 2011, up 17.4 percent from 65.2 million square feet in 2010. A majority of 21 of the 30 U.S. Central Business Districts in the report had an increase in new leases in 2011.

“We've hit bottom in a majority of cities in the U.S. and tenants took the initiative to either trade up or expand before rents increase substantially,” Maria Sicola, Cushman's executive managing director of research in the Americas, said Monday in a statement.

Total net absorption, the aggregate measure of new occupied and vacated space, increased to 12.9 million square feet in 2011, up from 2.2 million square feet in 2010.

The country's average office vacancy rate fell 0.3 percentage points to 13.5 percent. Three Manhattan office markets -- Midtown, Downtown and Midtown South -- and Portland, Ore., had the lowest vacancy rates in the country.

The average nationwide asking rent was $37.25 per square foot. Midtown Manhattan also had the highest average asking rent, $65.42 per square foot, up $1.35 from the previous quarter. Washington D.C. was the second-highest, at $49.70 per square foot, and Midtown South was third, at $45.90 per square foot.

Cushman's report echoed earlier, positive reports of office space, but continued growth in 2012 is dependent on larger economic factors.