Citigroup Global Markets hit the base metals sector with downgrades Monday on the belief that share prices in the group are nearing their peak.

Miners BHP ( BHP), Anglo American ( AAUK) and Rio Tinto ( RTP) were all reduced to hold ratings from buy.

"The sector is now sitting at, or very close to, all-time highs," states the report authored by Heath Jansen and his team at Citi in London. "The miners are now trading at about a 35% premium to net present values, which has historically been the right point to trim positions."

Shares of the three firms were all sliding between 2.5% and 3% in recent action in New York.

The report recommended sticking with Norsk Hydro ( NHY) and Xstrata, both of which are trading at a "discount to their peers."

Still, Norsk was losing 3% in U.S. trading, while Xstrata was off 3% in London.

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