ISTANBUL (Reuters) - The United States warned of the risks of countries manipulating their currencies during a meeting of G20 finance chiefs in Istanbul, a non-U.S. official who was present at the talks said on Tuesday.

"The Americans very precisely warned everybody that this is a risk ... that there are signs that there is currency manipulation going on," the official told reporters. "It was said not as an abstract possibility but as a real element of current reality."

A bipartisan group of U.S. lawmakers is expected to unveil legislation on Tuesday aimed at stopping trading partners from manipulating their exchange rates to gain a competitive edge.

The legislation, which is similar to bills introduced in previous years, would treat currency undervaluation in a similar way to subsidies provided by foreign governments and allow U.S. companies to seek compensatory duties on imported goods.