By Lewis Krauskopf

(Reuters) - U.S. stock index futures fell on Sunday amid news of North Korea's firing of four ballistic missiles and President Donald Trump's accusation that his predecessor, Barack Obama, wiretapped him.

U.S. stocks have climbed to all-time records, with the Dow piercing the 21,000 mark last week for the first time and the benchmark S&P 500 rising 11.4 percent since Trump's election last November.

Investors are upbeat about Trump's economic agenda, including tax reform and infrastructure spending, but wary of stock valuations that are historically expensive. The S&P 500 is trading at about 18 times forward earnings estimates against the long-term average of 15 times, according to Thomson Reuters data.

The former top U.S. intelligence official rejected Trump's accusation that Obama wiretapped him during the 2016 presidential campaign, even as the White House urged Congress to investigate the allegation, which Trump made on Saturday without offering supporting evidence.

North Korea fired four ballistic missiles, three of which landed in Japan's exclusive economic zone, Japanese Prime Minister Shinzo Abe said, the latest in a series of provocative tests by the reclusive state.

S&P 500 e-minis <EScv1> were down 7.75 points, or 0.33 percent, with 39,400 contracts changing hands, as of Sunday evening.

Nasdaq 100 e-minis <NQcv1> were down 14 points, or 0.26 percent, in volume of 7,210 contracts.

Dow e-minis <1YMcv1> were down 57 points, or 0.27 percent, with 7,035 contracts changing hands.

U.S. 10-year Treasury futures <TYv1> were last up about 0.14 percent in electronic trading.

(Reporting by Lewis Krauskopf in New York; Editing by Peter Cooney)