BBC News reports that the agency was sued in February over its assessment of mortgage-backed securities before the financial crisis.
The suit alleged that S&P turned a blind eye to risks in the products, and gave them ratings that were too high.
The US government is seeking $5bn (£3bn) in penalties.
In a court filing on Tuesday, S&P, which had previously said the case was without merit, claimed that it was being sued in retaliation for its downgrade of the US credit rating in 2011.
S&P downgraded the US rating in August 2011 by one notch, from AAA to AA+, amid a standoff in Congress over whether to raise the country's borrowing limit.
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