Sept 28 (Reuters) - The U.S. office vacancy remained flat for the third quarter in a row at 16.1 percent, real estate research firm Reis Inc said on Thursday.

Both asking and effective rents rose marginally at 0.3 percent from the second quarter.

"The vacancy rate could continue to stay flat as occupancy grows at or near the same pace as new completions just as it has over the last two years. This should keep a lid on rent growth over this year and next," Barbara Denham, senior economist at Reis, said in a statement.

Overall net absorption, which is measured in terms of available office space sold in the market during a certain time period, increased to 5.4 million square feet of office space in the third quarter, compared with 4.8 million square feet in the preceding quarter.

New construction of office space declined to about 6.5 million square feet, from about 10.4 million square feet.

New York City was the tightest market with vacancy rate at 8.6 percent, while Washington, D.C. had a vacancy rate of 9.3 percent. (Reporting by Ahmed Farhatha in Bengaluru; Editing by Maju Samuel)