Three initial public offerings were postponed on Tuesday, the latest casualties of volatile market conditions.

Offerings by Portuguese mobile marketing and payment services provider TIM w.e. SGPS SA, which was expected to raise $146.3 million, and 3-D motion-sensing company InvenSense Inc, due to raise $99.8 million, were postponed, underwriters said. The $180 million IPO of Seattle-based lender HomeStreet was also sidelined, an underwriter said.

U.S. stocks plummeted on Monday after Standard & Poor's cut the United States' AAA credit rating.

Although markets rebounded slightly on Tuesday, the benchmark S&P 500 index has dropped nearly 17 percent over the past two weeks on concerns in Washington over the debt ceiling and weak economic data.

IPOs are considered among the riskiest investments, and the IPO market often shuts down during a sell-off in the broader market.

Cathay Industrial Biotech Ltd pulled its $89.7 million IPO on Friday, and Enduro Royalty Trust put its $330 million IPO on hold on Monday.

The average run-of-the-mill deals that have to be sold, deals that have to be marketed and sold to investors ... those deals will probably slow down, said Morningnotes.com founder and IPO analyst Ben Holmes.

Even big government-backed IPOs such as Ally Financial's planned $6 billion share float have been delayed. The IPO, first expected in June, then considered for late summer, is now not expected until at least September.

So far, the IPOs of SandRidge Permian Trust, TrustWave, WageWorks, Carbonite, Midland States Bancorp, WhiteGlove Health and Loyalty Alliance Enterprise are still scheduled for this week, underwriters said.

(Reporting by Clare Baldwin; Editing by John Wallace, Maureen Bavdek and Richard Chang)