SAN FRANCISCO, CA: Google Inc extended an already wide lead in the U.S. Web search to 63 per cent share of the market in August, its biggest monthly gain in five months, a report said on Thursday.

Yahoo Inc, the No. 2 player in the U.S. Web search market saw its share of the business drop 0.9 per cent to 19.6 from July while Microsoft, the No. 3 U.S. player, slipped 0.6 per cent to 8.3 per cent, according to comScore Inc.

IAC InterActiveCorp's Ask.com grew 0.3 per cent to retain its fourth-place ranking while Time Warner Inc's AOL edged up 0.1 per cent to 4.3 per cent, according to August monthly data published by the market research firm said.

Google's growing share of Web search and, by extension, its even larger role in the related market for Web search advertising, has lead rivals and some industry trade groups to complain to competition regulators in the United States and Europe.

ComScore estimates that the number of searches performed by U.S. Web surfers on the five top search engines was virtually unchanged at 11.75 billion searches compared with July. The figure excludes searches users perform for mapping, local directory information or user-generated videos, it said.