WASHINGTON (Reuters) - Three of Washington's best-known monuments were defaced with graffiti over the Presidents Day weekend, the third major vandalism of memorials in the U.S. capital since 2013, a National Park Service spokesman said on Tuesday.

The Lincoln Memorial, Washington Monument and World War II Memorial were defaced with a marker pen sometime from Sunday night into Monday morning, spokesman Mike Litterst said. He did not give details of what was scrawled on the buildings.

"It doesn't appear to be politically or hate motivated," and the graffiti was not sexual in nature, Litterst said in a telephone interview. He said the markings each roughly covered the area of a hand with fingers outspread.

A monument preservation crew has done a first cleaning at the Lincoln Memorial, and the clean-up at the three sites will take a few weeks, Litterst said. The monuments on the National Mall are among the top tourist draws in Washington.

Litterst said vandalism of monuments on the Mall occurred occasionally. A woman splashed green paint on the base of the Lincoln Memorial in 2013, and in November last year graffiti was spray-painted on the World War II Memorial in protest against a pipeline project in North Dakota.

(Reporting by Ian Simpson; Editing by Frances Kerry)