The U.S. has entered a bear market and will stay there for a while thanks to a weak economy at home and a debt crisis in Europe that threatens to punish the global financial system, says investors Dennis Gartman.

"We are in an unmitigated bear market," Gartman, author of The Gartman Letter, tells CNBC.

The bad news gets worse, Gartman adds.

"The rally that we had last week took place on very low volume. There's further to go on the downside and there's still a deal of fluff left in the market that has to be taken out."

Loose monetary policies that recently pumped global economies full of money won't work this time around, other experts say, adding recession is unavoidable.

"We have done everything to monetary policy that we could do and this slowdown is going to be uncontrollable," Roger Nightingale, an economist and strategist of  RDN Associates, tells CNBC. "We have a serious recession coming, possibly even a depression."

Bond markets are also flying recession-warning flags as well.

"It is increasingly apparent to us that policy options are limited and that economic growth is slowing down," says Bill Gross, manager of Pimco, the world's largest bond fund, according to Reuters.

Low Treasury yields are indicating the slowdown.

"They certainly reflect, in terms of their yields, not only a potential for a recession but the almost high probability of recession and the result of lowering inflation," Gross says.